This was about a year ago, when i was still in school, i was 16 at the time.
It was a weekend and my really hot media studies teacher had asked us to come in specially to do some work on the school newspaper. i was getting really bored sat with everyone in the IT room typing, so i went for a walk round the empty school with a few mates.
On the way back i decided to go into one of the empty rooms as i needed the A3 printer in there. The rest of my mates went back to class and i sat in the room on my own. It was a hot day and for some reason i had a desperate urge for a wank. As no one was around, i thought, why not; no one knows i'm in here, so i pulled down my pants and started to do what we blokes do best.
I'd just come to the end when my media studies teacher walked into the room, wondering where i'd got to. she caught me there and then, holding my dick, all flustered, all alone in the classroom! She stood there and then said rather anoyed, "back in here please". i've never been so embarrassed and didnt live it down til school ended. It was almost the end of the school year anyway, but we could never look at each other in the same way again.