Here's a prank you can play on someone that I just thought of, it's quite silly and immature but fun for the few minutes it lasts. (Or if you pick a really dumb victim, it may go on for ages, in which case you may split your sides open with laughter). I actually just did it to myself by accident, and was almost fooled for a second lol.

1) Take a screenshot of your victims desktop, paste it and save.

2) Then, hide all icons on his/her desktop, and apply the screeny as the background pic.

3) Sit back and grin as they become confused, then angry, as they click away at the icons like a possessed person wondering why nothings happening!

At this point, you could make things even more interesting by saying you think you downloaded a virus or something. Then they shout at you, and maybe even turn a shade of green. They shouldn't figure it out unless they go into display settings (they may not even realise then lol)

It's the work of a genius

I should add, you don't need to hide all their icons, I did because the screeny I had was slightly different to my actual desktop. If the picture is identical to the background then it doesn't matter I guess. I just did it again with a new one and it works really well.

EDIT: Seems I didn't think it through a smuch as I should have done. DO hide the desktop icons, and if you can somehow hide the taskbar other than dragging it down or using auto hide, that makes it even better.

Ah yes, I used to do this back at school
They'd never be able to sort it out, good fun!

That is so cruel but funny
... But I know I wouldn't let anyone on my computer long enough to pull it off......don't touch my computer

Wow your school let you do a lot. Changing the wallpaper has always been disabled here, can hardly do anything lol.

The teachers were huge noobs, easy to trick!