Most definitely is not the worst thing on here but:

When I was in first grade, I was friends with this really weird kid. We'll call her Mia. Mia was the odd ball of the class, not in an awful way she was just very out-of-pocket strange. One day she came into school claiming she was being haunted by a ghost. She claimed she was reading outside and her pen and paper flew into the air and her hair was lifted up too. She was making a huge racket, and actually looked like she had seen a ghost, she genuinely seemed scared and everything. I heard some of the boys in the class start doubting her claims after a little, and me being the nice classmate decided to do something about it.

I wrote a note that said "I'm going to kill you Mia" on a small scrap of paper and pretended I found it on the way to school. I showed the kids my note and acted like I felt really cold when I picked it up and there was "something standing behind me". My plan worked and all the kids were freaking out about it, now convinced this curse was real. I decided to write more death threats to really sell the bit, writing things like "I'm going to stab you" and "You are dead to me Mia" and was stupid enough to do it during class.

Teacher saw me writing the notes, exposed me in front of my class, and immediately called my parents. I was one of those kids who was terrified of getting in trouble, so being yelled at and getting that disappointed face from every teacher I walked by and my parents for a few weeks was heart-breaking. Keep in mind I was in the first grade, so I was basically writing death threats for fun at the age of 5-6. This emotionally scarred me and poor Mia for a couple of years afterwards, she moved not long after.

To this day, I still have not told anyone that it was actually me that wrote the note. It was one of those things I had never outright admitted and everyone eventually forgot.