In Georgia its based on enrollment #'s, # of out of district attendees (city schools), and whether they are a private school or not.
Lots of debate on this yearly due to city schools with more out of district athletes being able to play down a class or 2 because their enrollment is low.
Public High School class designation is based on "BOY" enrollment and nothing else! This is because when u go back to the 50' and 60's when High School sports started there were no Women's sports.
So if ur school had say 70% women and the competing school had 30% women the school with MORE boys would obviously have a huge advantage cause they have far more boys to choose from to play a particular sport so to make it fair they based it on "BOY" enrollment that determines ur Class size.
Now today that doesn't matter as much as we have both Men and Women sports being played today in HS and the make up of Women to Men is very close but its still the standard in HS Class designation.