ive cleaned schools in the past. i still do one.
The main problem i had was when cleaning inside windows. there are paintings, etc usually stuck to the windows with blutac. not hard to get off, but annoying.
Also, i guess the work will need to be done quarterly. try and do the work on a school holiday, as you dont want any kids tripping over your hose, or bucket!
.... another thing. make sure you have someone with you if you are in a room with children, just incase, for your own safety!