This will be very difficult dude. Hacking the school server would get you kicked out. First of all, some schools (like mine) have HIGH security, so I don't recommend hacking/messing up their servers . You should get more information on your school before doing this.
Oh and if you have a Username and password to enter a PC, don't use yours. Find another way in, I did Anyways, the best thing to do is download some viruses/mess up the system settings/files so that the PC gets screwed up, then if you're lucky and did enough damage, and have added a virus to that PC, it could spread to the others.
Do some research on your school first, then do something like that. I know how to hack into my teachers school PC. I send her messages telling her off by a sort of javascript application and she doesn't know who it is