Pre-school in Japan lasts 3 years. So them being in the same school is normal.
Yeah, they also put them in formamide that is why they stay the same!
....
I was jk then, i am jk now.
What's formamide?
A chemical substance used in the crypreservation of organs....nah! I should say one that sounds more terryfying, like paraformaldehyde....mwuahahaha (evil laughter)
Oh! Formaldehyde! I was wondering if that was what you meant but I wasn't sure.
One year passed, everything changed, everyone moved on.... except the kids. They stay the same and in the same class.... :P
Don't get me wrong, i love this manga and the mangaka!