If you're looking for anime similar to 12-Sai: Chiccha na Mune no Tokimeki, you might like these titles.
11 year-old Sana Kurata is a positive, upbeat child actress with a rather odd life. She was adopted as a baby by an eccentric novelist who wears a live squirrel in her hair, her acting career is managed by a (former) homeless man who Sana unabashedly claims is her "gigalo", and at school her teacher has completely lost control of her classroom due to a group of blackmailing bullies led by "demon child" Akito Hayama. With her whole class in danger of failing, Sana decides she's had enough of Akito's nonsense at school and challenges him. The whole school is ready for a showdown as these strong personalities collide, but as the two very different kids learn more about each other, something unexpected happens. Is it possible for sworn enemies to become fast friends?
12-sai: Chiccha na Mune no Tokimeki and Kodomo no Omocha are shoujo series that have preteen female leads and have similar plot elements that younger female viewers will relate to and enjoy.
Honestly, I haven't seen an anime similar to 12-Sai yet but the most similar I've watched is Kodomo no Omocha. The boys are pretty similar I guess, they're stubborn at first. I can't really think of many similarities but probably guys fighting over the same girl is another one though out the story.
Puberty is an embarrassing, trying time for a child. Your body changes, you start to have feelings for the opposite sex, and more than anything, you are confused and bewildered about what’s happening to you. Tsubomi, Rea, Saya and a group of other fifth-grade students are being faced with this difficult event, and will experience the joys and horrors of it together. From learning about the difference between the male and female bodies, to experiencing a first kiss, to discovering the cause of pregnancy and even getting a first period, Tsubomi and her friends will finally learn what it means to be a grown up.
12-Sai: Chiccha na Mune no Tokimeki is about a group of 6th graders who are all experiencing new things, from a first kiss to first period.
Why it's similar: Both groups of characters are around the same age (6th grade). Both animes center around classmates trying to get through becoming an adult. (I don't know how to phrase that)
Tsubasa Sonokawa is a 15-year old high school girl who fell in love with her upperclassman when he saved her after she fainted at the train station from anemia. She thought it was a meeting of fate, and visited him at his class every day. However, he told her that her feelings were "heavy," and he dumped her. She once again meets a boy after taking over her friend Yuri's job as boy's basketball team manager. She learns that she might have more of a connection with this boy named Aki than she first thought.
Everyone knows that lies can come back to bite you, but when Erika makes up a fake boyfriend to impress her new classmates, the results may literally end up hounding her forever! Yes, it was dumb, but what were the odds that the random guy whose picture she used would turn out to be a student at her own school? Or that said faux-boyfriend's silence would come with such a horrible price? Now Erika doesn't just have to run and fetch at Kyoya Sata's command, she has to bark too!Because he's literally treating her like a dog, and she just has to roll over and take it, or he'll tell everyone the truth! But that's not the worst part, because even though this wolf in hunk's clothing has Erika collared, she may be starting to have REAL feelings for him anyway! Is she barking up the wrong tree, or could true love be unleashed in Wolf Girl & Black Prince?
The romantic comedy centers around Makoto Kisaragi, a 15-year-old girl whose "special ability" is her persecution complex. She was raised well but in relative obscurity compared to her big sister, a famous model. Despite her negative personality, Makoto vowed to become popular when she entered high school, but things did not turn out quite so well for her. One day, she meets Iriya, Mr. Popular in their class, and they become friends.
Short, athletic Hikaru Suzuki has just started his first year at junior high school, and things couldn’t be more eventful. He’s reunited with his sisterly childhood friend Chihiro Itou and had a glance at the wealthy, egotistical other Suzuki named Shinobu. However, most importantly, Hikaru’s first day of school yields another life-changing encounter: his first meeting with the shy would-be actress Sayaka Hoshino…
Welcome to Hijiri Academy! Completed only as of this year, freshmen are the only students who walk its halls; and now, a student body has been elected. As newly-appointed vice president, well-liked and cordial Midori Saejima knows she has her work cut out for her, in more ways than one: she’s been in love with the president, Akira Sudoh, ever since she secretly saw him care for a stray kitten one rainy night. As Midori’s feelings continue to grow, she finds herself more and more convinced that Akira has a girlfriend, much to her dismay. Will Midori come to terms with her feelings and confess, or accept that her love may be unrequited?