I never imagined it being anything but a boarding school. Why else would Jill have been behind the gym crying with no one around? Even during recess, I would imagine the teachers would want to know where the students were. Also, talk of escaping through the back gate would not be possible if one was only in the school for the day and then went home. The bullies also had plenty of time to "attend to" their victims outside of classes.
PotW said it right: it's obvious from context.
MrBob