Thread:AguriiMadoka/@comment-24288618-20180322015410/@comment-14175765-20180322022548

In Japan, children start elementary school after their 6th birthday, so by extension, they should be 12 years old when they enter middle school, 13 when they get to their second year and 14 their third year.

Assuming that the new school year begins at April 5, 2018 when the first episode airs, Ema would have to be born in 2004 for her to start her third year (approximately 14 years and 15 days old). Aine and Maika would then also be born in 2004, but in the summer after the April cut-off which is why they're a grade below Ema, and joining them is Mio who would be born in early 2005.

If Ema was born in 2003, then she'd already be in high school.