pg 37-38... she like, came a little, amirite? I've picked my wife up plenty of times like that when we were young... can't say it's something I've observed IRL
@alfaindomart It has to do with when the school year starts. Japan schools start in spring, the US start around september(I think it easier to use this for an example).
Let's say the year is 2010(easy math). He turned 15 in Aug '10. School started Sept 1st. She wouldn't turn 15 until her birthday the next year, Feb '11. When the school year ends, in say March, both of them will be 15, but she was still born the year after he was.
I guess that if the school year starts in april, everyone born april to march end up in the same grade, having her born in the next calendar year but still being classmates.