Dumbest thing ever would be... hmm... I know!
Not asking to switch to Biology 9th in my Freshman year of High School. So I wasted a year learning nothing since I already knew everything. Not exactly good, since I'm switching to IB.
That, or the time I put saliva on someone's hand because they asked me to. Long story. So, let's get on that story.
Sub day in Geography. There was a new kid that week. He kept on putting his feet on desks on chairs, and I kept taking them off because his feet were dirty and no one likes sitting in dirt or putting their papers on dirty desks.
So, I licked my hand, and held it up. "Come on, do it," he said. So I did it later near the end of class. Only licked on finger and touched the back of his hand, and the bathroom was 10 steps away from our classroom's door.
"Why'd you do that?" I didn't respond because that was a stupid question. He asked me to.
BAM, punch. He then ran off. I then stood there grinning. "OMG he's bleeding!" a girl said 20 seconds later. I covered my mouth with my hand then, a teacher came in, etc.
By a miracle, the tooth was knocked out whole, so it could be put back in. The root canal they decided to do wasn't fun, I'll admit that.
BTW that kid got arrested for assault, either 3rd or 5th degree, I never followed up on what they decided to give him exactly, just that those were the two they were deciding between.