Several things: i once crushed another kids pet grasshopper, because insects can't be pets or some nonsense internal logic like that. He brought it to school all proud like, talking to everyone about it, showing it off to everyone who would look (so just the boys). So i crushed it in my tiny hands and then wiped it off on the nearest kids shirt.
Another time I copied answers from someone in a test and then reported her for copying from me. She normally doesn't do too well on tests but there was some video game or other she wanted to get and her birthday was coming up so she struck a deal with her parents that if she studied hard and did well on this test they'd get it for her. She'd been talking about it all week, and she studied hard for it and everything. Anyway, i copied her answers with better handwriting except for the last few questions (which i solved first) and turned my paper in before her and said to the teacher that i thought she was looking at my paper. She got marked as a cheater and ended up getting in trouble. This was third grade.
Once in an assembly in the auditorium I spilled a water bottle just after the door where nobody would see it while walking in, causing a whole bunch of kids to slip and fall on top of each other when they were walking in in a line, as primary schools tend to make you do. It only delayed the assembly by 2 minutes though, so it wasnt what i would consider a successful operation.
One time this boy was kinda into me so i grabbed spaghetti from his lunch tray in the cafeteria when nobody but him was looking and I squished it into my hair and started fake crying (which I'm convinced nobody can differentiate from actual kid crying, with all the stuff I've gotten away with). When a teacher came to see what was wrong I said the boy said he wanted to see if my hair was longer than the spaghetti. He got in trouble, I got to go to the nurses office to clean up, and I got to miss a grammar worksheet. 4th grade. This one was a success.
But the most horrible thing I did once was the one time in creative writing class when I wrote stories about horrible things I've done in school, but they were all in fact lies and it wasn't actually a creative writing class but this very askreddit thread and what I'm trying to say is none of these are true and i don't regret anything, except maybe the one about the grasshopper.
tl:dr i did some things I'm not proud of