Ok, school computer teachers (high school or under) usually set a few options and think the computers are all "secure." First, some easy ways to bypass certain locks:
To get into C: drive, or any drive, go into MS Word and type a few gibberish letters. Highlight it and press Ctrl+K. This lets you use hyperlinks, so just link it to "C:\" (without the quotes) and then click on the link it makes. It'll open C Drive in Windows Explorer. Now you can look at everything in C and other drives.
Some schools have C drive secure, so that if you try to delete or save anything on any place other than your own space, it goes back to normal next time you restart. Now, open windows explorer (you might have to use the technique in the last paragraph) and look at the other drives. There should be more than just C and your space, and chances are these aren't secure. For example, at my school, X: drive also contains some files that certain programs need but it isn't secure, so i could just delete it all and it would get deleted from the whole network.
If you're locked out of changing the wallpaper, open the picture you want in Paint and go to File > Set as Wallpaper.
I had a great idea the other day for if you like to play games and other things when in the lab when you're supposed to be working. A FAKE taskbar (the place at the bottom where the start menu and all the open programs are)! First, open the thing you were supposed to be doing and minimize it, so it should be the regular school wallpaper with the thing you were supposed to be doing in the taskbar. Press Print Screen on your keyboard (this copies the screen to the clipboard). Now, go to MS Paint and press Ctrl+V. Your screen should paste in Paint! Save it as something that looks like schoolwork (like project.bmp). Now Set it as the wallpaper (File > Set as Wallpaper Centered). Now close paint, it should look like it did before, but if you click on the top of the taskbar and drag it to the bottom of the screen it should disappear and show the fake one in its place! Now you can open the internet or game or whatever and press Alt+Tab to switch back to the other thing when the teacher comes close, and all he/she'll see is your work with the fake start menu at the bottom. You're welcome. :D
If you like what you see rate me high and i'll let you in on some more secrets.