Our school have some kind of a system. I guess it wouldn't be hard to hack the passwords, I'm sure they're something obvious, but I think the grading computers are on a separate network...
I did do some mischief back when I was in Elementary School (11 y/o). The Teacher Union's forum wasn't too secure to say the least, the password being 2003, and you can choose any username you want
So we looked up the admin, and made a fake post about the forum about to go down for a long time or so (hey, we were small kids, we couldn't invent anything better!). Eventually they had to explain to everyone that the forum wasn't really closing... Heh, that was funny.