What a giggle this thread is and some of the examples could have come straight out of a Just William story! 
The worst thing I did is known as The Incident of the Milky Ways in our family. At my primary school, Primary 6 ran the school tuck shop. It sold small 'penny tray' things like Black Jacks and Dainties, right up to proper chocolate bars like Galaxy Ripples (my favourite), and of course Milky Ways.
When my class got to Primary 6, we proceeded to eat our way through the stock, paying a fraction of the correct price, or giving customers we liked more sweets for their money. It all went wrong when the son of one of the teachers started school and some bozo one of us (not me) gave the little boy too many sweets for his money and his Mum noticed. One of the reasons the child was given too much for his money was the fact his Granny was our teacher, and the bozo thought it was a nice thing to do for our teacher's grandson. Yes, we had teachers who were mother and daughter.
Well, all hell broke loose. A stock check showed hundreds of pounds worth of stuff was missing. The headmaster came to our classroom and at that point I realised we were in serious trouble. Although only a group of us served in the shop, everyone was involved and looking back now I'm surprised nobody gave the game away, but I honestly don't think we realised that what we were doing was theft. After the headmaster spoke we were left in no doubt and scared to death. We had to make a list of things we hadn't paid for and letters were sent to our parents asking for the money. We'd been doing it for months, so of course we couldn't remember and just made it up. I think my list came to about 7 shillings - a fortune to me at 11 years old.
The shame. My delightful little brother (aged 9) who was at the same school and knew what had happened, proceeded to blackmail me for sweets and favours for not telling my parents before the letter arrived. The shame when the letter arrived stays with me to this day. I let my parents down. I probably got a hiding, but it's the shame I remember.
My brothers and I giggle about it now, but it's never mentioned in front of my Mum.