Just a little addition:
Being on your school's network, its not just a "Its my network and I can do what I want with it". You also have to remember that on whatever network you are connected too, something leads back to your provider. While not the same thing with companies providing internet service, if say, your school gave you access through their network and unknowingly allowed you to post "hateful" (You know what I mean) things, it could(however unlikely) be misunderstood as their support for whatever you said. Kinda covering their butt thing :-p
Important about those locker and/or car searches is that the school isn't just looking to bust someone, they're looking out for however many students could be affected if one did have something dangerous on campus.
In my own arguments pre-emptive defence, the difference between this and, say, searching your house, is there are only so many "personal" things you could possibly have in your locker, compared to your home. :-p
*edit*
Humorous note: A drug dog once dug into a girl's bag, only to eat her peanutbutter and jelly sandwich
