As a 7-8 year old got a girl to touch a hot pile of glue from a glue-gun. Someone else had done it to me a few moments before and I reacted instantly, wiped it off on the corner of the desk before it could burn me, but the girl didn't have the same instincts so just let it burn her whilst screaming.
Some petty theft from the school tuck shop when I was around 12-13 (got caught).
Set fire to a number of Science journals which were kept just above the gas taps in out science classroom. I thought it'd be funny for a few books to go up but it quickly got way out of hand requiring multiple fire extinguishers.
Was the "armourer" during our inter-school fights against the other local high school when I was 16. I'd carry a few metal bars in my huge school bag incase we started to lose any of the many fights we had before the Police cracked down on it all.
Looking back, even though I didn't consider myself a "bad lad" at the time, I was not a nice kid and it took a few years in the Military to truly sort myself out.
My worst trait however was that I was a smart-arse to the teachers involved in lessons I didn't care about (RE, Home Economics, Sewing, Spanish/French etc) and I regularly spent whole days sat outside classrooms because the teachers couldn't handle being spoken to as an adult by a child, so as a "disruptive" child I was left outside the classroom instead i.e. telling the sewing teacher "just show me how to sow a button, darn a sock, patch a pair of trousers but nothing else as the rest is wasted on me and I don't want to know it. I'd rather you spent your time teaching those who actually wanted to learn from you, cheers!" etc which when it's coming from an 11 year old can be hard to deal with!