How to make a background

First thing first, find a page that is allowed on school servers.

Pages I would recommend: http://www.spacetweaks.net/layouts/space/index.php. or http://www.layouts.fm/

Copy the code under the background you want, then go under widgets, and go to the very bottom "other html" one and paste the code. Hit save and your good to go!

Remove those little banner things at the top of the page, paste the code into notepad, and take off the first two lines. there should be a space that seperates the two sections, the bottom one is the background, the top is the adds. delete the ads, and paste the code as previously instructed.