Only once, more by accident than design. I was day dreaming as the RE lesson was very boring, I could feel the strain of my cock against by trousers and made an adjustment. This made my cock throb which to made me try to hold my cock down- The movement must have caught my mate's eye, who was sat next door to me, he sniggered, because my cock was very obvious.
This must have turned him on, because, he was filling our his trousers, we had both sort of lost interest in the lesson completely at this stage and very both very conscious of our cocks both now throbbing away in our trousers, me holding mine against my leg, him in front of him peeking over the top of his trousers, under his shirt.
Two horny guys sat at the back of the class, with hard cocks. I changed from suppressing to gently rubbing to see what my mate did, he had a fist full of his cock, and must have forgotten where he was (or didn't care) because he was wanking away . It caused me to really throb up and down with the excitement, and holding my cock was no longer an option, precum was visible on my trousers and I knew it could not hold on for long without blowing my load. My mate must have been nearly there also and as I glanced over I saw a shot leap up his chest in his short, soaking it.
I knew that I was not far behind and quickly tried to get a handkerchief down my trouser leg before it was too late. The movement of this just made it worse and I shot my load down my leg soaking leg down to my knee where it gathered - I could smell the cum from both of us. I'm am sure others could.
The bell rang for end of lesson and my mate and I slowly made out to gather our books although we were trying to buy time to clean up a bit.
Luckily for my mate - he had a jacket he could put on, I had cum all down my leg and knee - a eventually walked out of class with my jack held casually in front on me. Thank god for the ran outside, I made a fake trip and fell into a puddle which soaked my trousers , rolling slightly when I got up made me more wet, and less obvious I had cum down my leg.
Not something I repeated, my mate and I used to laught about it, but I certainly never repeated it.