When I was 16 and a Junior in High School I had a good reputation for being a reliable kid and good student. So the Phys Ed teacher just naturally selected me to assist him. My job was that on Mondays I had to wheel out a cart of towels into a tiled area by the showers and as the boys came out of the shower they came over to the cart and I handed them a towel to be used for the whole week, and I checked their name on a list. Then on Fridays I wheeled in a bin and collected the towels from each boy after they towelled off, and checked their name off the list. I was allowed to go to my next class late so I would have time to shower after all the towels were taken care of. The job gave me a great opportunity to see all the boys completely naked. It also gave me a chance to see them fooling around together in the open showers. Even though there was no actual jerking off there was plenty of slapping, teasing, sexual pranks, horniness, sometimes gropes, and a lot of boners every day.
One thing I learned was that if one day a few boys start to get boners, it's contagious and most of the boys wound up with one or at least half a one. I had on my jock and gym trunks the whole time so the fact that watching gave me huge boners every time was my own secret. After the first few weeks, I got up the courage so that when I went into the shower alone after everyone else left, I would go to the back and jerk off. I also used the info I had observed and noted one really cool kid who had a big boner all the time and was aggressive in the showers in sexual ways, making jerk off jokes and fooling around with the others, and I worked to try to get to know him better. That worked, and we became really good friends. We had a lot of experiences together starting late in my Junior year and continuing sometimes even now.