Alright, here's a really evil one i did:
We had a guy in our class who was completely controlled by his feelings and he was plain easy to manipulate. He was in love with a girl in the class (who in my opinion is an ugly [bad word] but everyone seems to think she's pretty). So we made a fake MSN of that girl, and we asked her to give the victim her "new" MSN adress. On a side note here, almost everyone in-and outside of the class knew about this prank BUT the victim. I find it hard to be so unlucky or dumb not to hear it because me and my friends were really nervous about people always talking