I’m lost I just started this...they’re 19/20 years old in HIGH SCHOOL?

I haven't read this but in east asia they add 1 year to the age, so that you'll never be 0 years old. so you are born as 1 year old, and then after your first birthday (or the year after the one you were born in) you'll be 2 years old. This is also why you are of age from 19 in korea, and not 18. so idk if that's the case but I hope it helped