School's Back in Session! See Which Celeb Kids Are Gearing Up for Their First Day

Harper Renn may have a future career as a marine biologist! Tiffani Thiessen's 8-year-old daughter is all set to start third grade, with a "Narwhal Nirvana" backpack by Ju-Ju-Be.

"First day of 3rd grade. And bittersweet it really is. My baby girl is growing up so fast," the Dinner at Tiffani's star wrote.

Holt Fisher, 3, might be even more excited than his big sister Harper about starting school, holding on to a Tula Kids Backpack in "Chomp" ($79) and a sign expressing his love for Star Wars.

"Holt's first day back! Year 2 at preschool. This kid is a ham and keeps us laughing daily," Thiessen captioned a photo gallery of her ecstatic baby boy.

Peyton Marie, 4, looks so grown up for her first day of preschool, smiling widely for the camera in a purple Cinderella shirt and pink backpack.

"Today is Peyton's first day of Pre-K!💞 I can't believe she's already here," wrote mom Victoria Prince. "I am so proud of my sweet baby girl!! 🤗"

Peyton's older sister, 7-year-old Jordan Kay, threw up two fingers - not as a peace sign, but as in indicator that she was heading to second grade!

Prince captioned her photo gallery, "My big girl started 2nd grade today!💞 And she was soooo happy after school that she had the 'coolest' backpack in class 🤗! #JordyBear"

Armie and Elizabeth Chambers' older child, daughter Harper, is all decked out in a blue-and-red dress with a Peter Pan collar and a red bow to start her second year of preschool.

"First day of school and just a little excited 🍎," Chambers Hammer joked, sharing a photo of her smiley 3½-year-old.

School-ready siblings! Savannah Guthrie's adorable minis Charles "Charley" Max, 21 months, and Vale, 4, share a sweet hug before their first day.

"My heart bursts (and breaks) to look at these two headed off to their first day of school!" the Today co-host captioned her photo proudly. "Thanking God this day and every day for my little dreams come true."

Molly Sullivan just turned 2 this summer but she's already a fan of school, according to mom Ali Fedotowsky-Manno.

"Molly did so great! I sat inside the classroom while she played outside with her new friends and she couldn't have cared less that I was there. I'm so proud!" the former Bachelorette captioned a photo of her older child holding a sign reading, "Molly' FIrst Day of Pre K 2018."

"I can't believe I get to post one of these 'look at my kid holding a sign saying it's the first day of school' pics! 😂" the star wrote.

Hilaria Baldwin's oldest child is off to kindergarten! Lucky Carmen Gabriela, 5, had dinner out with her mom and dad Alec Baldwin (and without her three brothers!) on the night before her first day of school.

"1st day! #CarmenGabriela," Hilaria captioned an image of her daughter holding a sign bearing her name and the message, "1ST DAY OF KINDERGARTEN!"

Following big sister Carmen's lead, the Baldwins' oldest son Rafael Thomas, 3, is heading off on his first day of pre-K!

"First day!!! ❤️ #RafaelThomas," Hilaria captioned a snapshot slideshow of her little boy holding a sign with his name, the milestone explanation and the year.

The second photo showed a casually dressed Rafael excitedly holding the sign high up in the air and sporting a colorful backpack.

Harper Grace and Gideon Scott may be twins, but Neil Patrick Harris' cuties, who turn 8 years old next month, are complete individuals in their first-day-of-school outfits.

"First day of school! Gideon and Harper are officially second graders!" Harris captioned the cute photo (taking by husband David Burtka), adding the hashtags "#proud," "#papa" and "timeflies."

Daphne Oz's two older children - Jovan Jr., 3 next month, and Philomena Bijou, 4½ - are hitting milestones right before their mom's eyes.

"Oh hey, grownups 😍😩😍," the former co-host of The Chew wrote, sharing a photo of the sweet siblings before their first day of school.

Alena Rose is ready for pre-K! Kevin Jonas' 4½-year-old daughter can't contain her excitement for her first day, smiling while posing alone and with her little sister Valentina Angelina, 2 in October.

"First day of Pre-K 🙈😭 #lategram," mom Danielle Jonas wrote alongside the sweet sibling photo set.

Vanessa and Nick Lachey's 3½-year-old daughter Brooklyn Elisabeth was adorably pouty in an honest photo her mom posted to Instagram, sitting on the family's front stoop with her head down and her hands between her knees.

Posing next to a sign reading, "Brooklyn's 1st day of 4 yr. old preschool" alongside the date, the little girl looked ready for a day of learning regardless of her expression, wearing a cute bunny-pattern outfit, a red bow, gold Mary Janes and socks with a pencil-like design.

"We're both NOT happy about this!!! 😩🎒📚 #BrooklynElisabeth," Vanessa captioned the memorable snapshot, also showing off Brooklyn's sweet (and affordable!) unicorn backpack by Skip Hop ($17.50).

Decked out in their academic best, Jessica Alba's daughters Haven Garner, 7, and Honor Marie, 10, were all smiles as they posed together before heading off to their first day of school.

Honor rocked a stylish hairdo, with fun highlights on the ends of her locks. She also wore camouflage pants, a colorful jacket and the Emily & Meritt Olive Studs Backpack by PBteen ($69.50). Younger sister Haven sported pigtails and the Mackenzie Blue Glow-in-the-Dark Mariposa Backpack from Pottery Barn Kids ($34.50 to $44.50).

"My babies all grown 💔 😫," wrote Alba to accompany the snapshot, tacking on the hashtags, "#5thgrade," "#2ndgrade," "#schoolsbackinsession" and "babygirls."

Academia is a magical force for Meilani Alexandra!

Jenni "JWoww" Farley's 4-year-old daughter looks as proud as can be in her unicorn T-shirt, rainbow-covered pants and Disney Princess backpack.

The Jersey Shore star wrote alongside the image of her older child, "She's ready for her first day of preschool 😩😩😩😩😩😩 #imnotready"

Another pair of mini meatballs! Adorable siblings Giovanna Marie, 4 this month, and Lorenzo Dominic, 6, are dressed casually for their big day.

"It's happening. Run free my tan little nuggets," Nicole "Snooki" Polizzi captioned the pic of her kids.

She added, "Mommy & Daddy are so proud of you! Make friends, learn, & be great fricken people. 👧🏽👦🏽❤️💯"

James Wilkie, 15, is ready for his second year of high school in this photo shared by mom Sarah Jessica Parker.

"Our boy. Our sophmore. All our hopes that his academic year ahead is adventurous, challenging, enlightening and fun," the actress wrote.

"So carpe diem my latin student! X, SJ (mama)," she added, giving husband Matthew Broderick a shout-out for capturing the memorable moment.

Parker and Broderick's twins Tabitha Hodge and Marion Loretta Elwell, 9, headed back to school in style - Tabitha was prepared with her The Meadow backpack by Parkland ($50), while Loretta opted for the brand's Vintage Blue Dragon backpack ($40).

"And the last of the fold. Our brunette. The 'baby' embarking on her first day of 4th grade," Parker wrote on her photo share.

"Small but mighty, her knapsack prepared days ago, she take long strides toward a whole new year awaiting you Tabitha. X, mama."

Pink-clad sisters Andy Rose, 3, and Pearl Clementine, 6, can't contain their smiles as they pose together before a day of fun-filled learning.

"First day of school! 💕💕💕" mom Lisa Osbourne captioned the cute pic.

School's in session for the littlest Willis sisters!

In an Instagram snapshot, Bruce Willis and wife Emma's adorable daughters Evelyn Penn, 4, and Mabel Ray, 6, are too cute posing on their first day of pre-K and first grade, respectively.

Both girls are wearing Mary Jane-style shoes (red for Mabel, silver for Evelyn) and navy-blue ensembles, opting for backpacks with pink straps for their learning escapades.

Emma captioned the sweet photo, "And they're off! ❤️ #firstgrader #prekindergarten #backtoschool ✏️📚"

Mario Lopez sent his younger child, son Dominic Luciano (who turns 5 this month), off to pre-K with some hilarious well wishes for his instructor.

"He's all yours teacher! Good luck ... " the star wrote, sharing a snapshot of his son holding up a sign listing facts about himself.

"The bubs first full days of school started today," Scott Porter captioned a photo of himself, wife Kelsey and their two kids: Clover Ash, 1, and son McCoy Lee, 3.

He continued, "I know #BigBoyMcCoy has been his tag for a while but he's getting TOO big. Time. Where does it go?"

Brooks Alan, 6, looks so grown up in a white polo shirt and black slacks, heading to kindergarten while mom Molly Sims snapped a couple shots.

"First day of Kinder 🍎❤️🙊 And so it begins ... momma cried ... a lot 😭 #firstdayofschool," she wrote.

Another pre-K entrant? Marlowe Mae! Eva Amurri Martino's adorable 4-year-old wore her hair in two pigtail buns, adorned with green bows, and an "ABC" striped dress on her first day.

"Ready, Set, GO! 🚦📚✏️ Marlowe's first day of Pre-K and she couldn't be more excited," the actress and lifestyle blogger captioned her photo.

"In fact her exact words this morning were: 'Is it finally my back to school day, Mom?!' Mom is also over here like 😜🙌🏻👌🏼🙏🏼" Amurri Martino added.

Samuel Thomas, 2, is too cute in a striped shirt and denim overalls, holding a tiny lunchbox as he heads off for the day.

"Maybe by the end of your first day of pre-preschool you'll be able to read this caption: I LOVE YOU SAMUEL BOY," proud mama Catherine Giudici Lowe wrote alongside her snapshot.

Ryan Carson has some back-to-school sass! Steph and Ayesha Curry's 3-year-old middle child tilted her head to the side and placed her hands on her hips for her mom's captured moment.

"Baby girl stepped into pre-k today determined, ready to learn and beyond excited to win over the hearts of her sisters friends," Ayesha wrote. "Love my sassy girl. 3 going on 13 😂💁🏽‍♀️❤️"

Proud dad Wiz Khalifa sent his son Sebastian Taylor, 5½, off to kindergarten with a big rite of passage: riding the school bus!

In a video, Khalifa defended his actions, addressing some of his naysayers (a.k.a. "All y'all people asking why I would let my son ride the bus, cause I'm rich.")

"Being rich don't got nothing to do with a child's experiences," the rapper said in the clip. "He said he wanted to ride the bus with his friends, so let kids do what they want to do. Chill."

"Back to school! First day of 1st grade," Roselyn Sanchez's husband, Eric Winter, wrote on Instagram. "She's officially a big kid! She's ready to tackle anything. Go get em baby girl!"

"Yes. I cried!! She went happily! I have two kiddos in school. Where has the time gone?!" wrote mom Tamera Mowry Housley.

"First week of school," Chrissy Teigen wrote of 2-year-old daughter Luna's big milestone. "They had us make a little family photo album she can look at when she's sad or upset. She loves it."

Emily Maynard's 13-year-old daughter is ready for the seventh grade! "She's probably way too old for this 'first day of school' board but I don't really care," the Bachelorette alumna wrote on Instagram. "She'll always be my baby, even if she's taller than me."

Kristen Bell and Dax Shepard's 5-year-old daughter holds her parents' hands as she heads to her first day of school.

Bachelorette alumna Ashley Hebert Rosenbaum celebrates her kids' first day of school with a "mom party."

"First day of school for these two delicious dishes! I don't know why but watching them get on the bus and drive away always makes me cry a little," Katherine Heigl wrote on Instagram of her daughters' first day back to school. "Then the bus pulls out of sight, I dry my eyes, pour myself a big glass of champagne and shout 'CHEERS TO SEVEN WHOLE HOURS WITH ONLY ONE KID NEEDING MY ATTENTION!!'"

Michael Phelps' 2-year-old son happily poses on his first day of school, with his backpack and lunchbox in tow.

Jamie Lynn Spears' eldest daughter, 10, slips into her fifth grade uniform.

Aja Volkman's 6-year-old daughter strikes a sassy pose for mom as she gets ready for her first day of first grade.

Mario Lopez's daughter is all smiles as she gears up for second grade.

Tamera Mowry's son, 5, is one cool kid as he embarks on a new milestone. "He couldn't wait to meet all his new friends," the Sister, Sister star wrote on Instagram. "He simply gave mommy and daddy a kiss and said: 'Goodbye Mom and Dad. Love you.' Me on the other hand, I had to hold back the tears." 

Ayesha and Stephen Curry's daughter, 6, is a "big girl" now that's ready to take on the first grade.

"Jolie's first day of school," country crooner Jana Kramer wrote on Instagram of her 2-year-old daughter's first day.

Melissa Rycroft Strickland's little girl is all smiles as she poses for a mother-daughter pic ahead of her first day of second grade.

Sarah Michelle Gellar and Freddie Prinze Jr.'s son Rocky, 6 next month, and daughter Charlotte, 9 next month, are ready to learn in coordinating uniforms.

In a photo shared to Gellar's Instagram account, the youngsters hold signs in front of their face that declare they are entering kindergarten and third grad, respectively.

Cricket Pearl, 5, and Birdie Leigh, 10, are all smiles on their first day of school.

"First day of kindergarten and fourth. I don't really understand how my kids are so big but here we are. ❤️😭," proud mom Busy Philipps captioned a photo of the sweet sisters sharing a hug.

Xander Dane, 7 next month, is all about the dab in a snapshot mom January Jones posted to Instagram.

Dressed in dark slacks, a red shirt and a blue jacket, the little boy - whom Jones rarely shares images of - covers his face with his arm and holds up a chalkboard sign reading, "FIRST DAY of 1st grade 2018."

"It's official, my first born is now off to his first day of Kindergarten," Vanessa Lachey began her lengthy caption on a pair of photos she shared to Instagram, showing her hugging her oldest child in the first snapshot and a second of him posing next to a sign that read, "Camden's 1st day of kindergarten," alongside the date.

"Camden John Lachey, the world is your oyster! To the moon and back and back again!!! Forever & Always... my baby boy!!! 💫❤️💋" Vanessa ended the tribute to her son, who turns 6 in September.