There's already a high school "Catfish" story here (the one engineered by my friends and I ended in a park at 2 AM, though), so I guess I'll go with the butt yogurt story.
When I was on 9th or 10th grade, a new kid joined our class and a bunch of us decided to mess with him. We started telling him about how we'd go out on weekends, get shitfaced, and fuck prostitutes (I'd never even tasted alcohol and I'm pretty sure we were all virgins). He was just sort of nervously laughing, not really buying it, but then another kid in our class (a friend of mine) came over and was like "why didn't you guys tell me about this?!" I tried to loop him in by "reminding" him how he came with us the last time and we totally fucked some bitches, but he just went "No, you didn't invite me, fucking traitors! Come on, I wanna have sex!!!" Slowly, the conversation shifted from messing with the new kid (he probably got weirded out and wandered away) to messing with my friend, who just couldn't believe we'd never told him about our sexual adventures.
This continued for a few more weeks: on Mondays we'd talk about how hot the prostitutes were and he'd get angry that we didn't let him come along. He was shorter than the rest of us, so I think we just told him he'd make it too obvious that we were kids.
Then things started to get slightly cruel. One time, one of us pretended to be really depressed all day until my friend asked what was going on. We told him he found out that the prostitute he had sex with last night had AIDS and he didn't wear a condom. I remember my friends' eyes getting a little wet. He went over to the other kid, opened his wallet, and gave him... I can't remember if it was $1 or a Pokemon card. Possibly both. Later, the other kid laughed his ass off and admitted it was all a joke. My friend went: "You fuckers! OK, so when are you letting me come with you?" The dumb motherfucker left us no choice but to escalate.
So, we told him he could come along on the next weekend, but he had to buy his own condoms (he actually did bring condoms, but maybe he just stole them from his brother). We met up at someone's house for the "pregame", because we insisted that we all had to be slightly drunk to have the courage to approach the prostitutes. The plan here was to get him drunk on beer (I just drank Fanta and pretended it had beer in it) until he passed out, but it took longer than we thought, so we actually went out for a while at the dangerously late hour of 11:30 PM, walked in circles, and said we had to go back because someone had forgotten to bring money. He kept shouting "I'M GONNA FUCK!!!!!" on the street. Even as he puked in the toilet, he'd mumble "...I'm gonna fuck..."
When he finally went down on the couch, we proceeded with the next step of the plan: pulling down his pants and putting yogurt on his butt and undies. The idea was that the next morning we'd all brag about how hot our prostitutes were and someone would mention that the one my friend with kinda looked like a man... but that never happened, because he was gone by the time I got up. Apparently he locked himself in the bathroom for like half an hour (someone said they heard crying, but of course they did) and then just left without talking to anyone.
He was quiet at school after that and I never asked him about that night because I felt too guilty, but I think he just slowly realized it was a prank and went back to normal. I hope, anyway. I'm shocked he still talks to me.