Ideas for the enemy groups:
Terrorists (although this is rather prevalent nowadays)
School gunmen (the "He was always so quiet..." kind of gunmen)
Religious wackjobs (may be similar to Terrorists)
Goverment Agency (maybe a proffesor was running a secret project or something? Actually I like that idea. That could be expanded really well, although you would have to try not to copy Back to the Future with the whole, "I stole the materials and built my own thing!" kind of idea...)
And ya, lockers, bathroom stalls, outside portable classrooms (my school has classroom outside that are essentially re-fitted cargo train freight containers with windows and furnishings. They aren't made from freight containers, but that's the closest thing I can think of to compare them to something that people would get) would all make for good hiding places.
Although where to get weapons in a school would be a problem to figure out... Obviously you could steal weapons from the enemies, but where to get your initial weapons would be problematic. Probably shop class, grab a big wrench or a screwdriver. Maybe a hacksaw...
You could have the main character was out for a run with his/her gym class and as such was not contained with the rest of the school.
I can really see how this would work actually. It would certainly be different from most of the games out in the market right now.
And if you read all of that, thank you very much!