How long does it normally take for a girl to reply to a text from a guy at school?

Right after she is done with her homework, washing her hair, walking the dog, filling the gas tank, moving the clothes from washer to dryer, starting a new load, folding the dry clothes and gets back from her dates.
Maybe a little sooner... but that is the norm.

Sounds like your really into this girl which is cool. But to answer your question it depends on the girl, maybe she's busy at work or something else; just give it time.