I was in Year 9, aged 13-14. I did taekwondo for a couple of years, which involved a lot of stretching, and I was very good at the stretching part, One of the stretches was the butterfly stretch. To explain it: you sit on the floor with a straight back, bend your knees and press the soles of your feet together, pull your feet close to your body, hold your feet or ankles with your hands, and gently press your knees toward the floor. You can use your elbows to push your legs down gently for a deeper stretch.
At the time, I could do this almost perfectly and get my knees flat on the floor.
I was sat on the school field at lunchtime with two girls, one of whom is still my girlfriend. For whatever genius reason, I decided to demonstrate the butterfly stretch after bragging about how flexible I was. It turned out I was more flexible than my school trousers, because as I was doing the stretch my school trousers completely burst open at the crotch and split down the seams on the inside of my leg.
For me it all happened in slow motion. I looked down and saw my pants were completely exposed as well as the top of my legs. I looked up and saw the girls laughing. I put my two hands over the middle to cover my pants but then I moved my hands to the side to try and cover my legs, and when I did that the girls laughed even louder. Then I went to uncross my legs and I felt my trousers split even more up the back. Basically, my trousers had split from the fly all the way up to the belt loops on the back, and down the insides of the legs.
I didn't have PE that day, so I was actually wearing white pants
It was really embarrassing, but at least it happened in front of two good friends.At this point I started to get nervous because I knew I'd have to go to the nurse to get a new pair of school trousers, and that meant walking back across the school field. I was really worried that my older brother, or one of his friends, or any of my friends, or just anyone would see what had happened. One good yank and you could have ripped the trousers clean off, I think.
By some miracle, we made it across the school field and to the nurse's room. A few people did see, but luckily no one made any attempt to damage my trousers any further.
The next embarrassing thing was dealing with the school nurse. She obviously saw that I was wearing white pants and then I had to try on three different pairs of school trousers.
The first were too small. The second were too big. The third pair, which I wore for the rest of the day, were very strange. I'm not sure if they were unisex or girls trousers
I remember they were kind of tight around the crotch but roomy at the hips. They also had popper buttons instead of a zip fly, I remember thinking. I bet it would be easy to pantsed someone who's wearing trousers like this. 
I also remember the girls light-heartedly teasing me afterwards, saying they could see the lines of my pants through the trousers, which they had never said before.
Hope you like the story, happy to answer questions or discuss further.
