This just happened a few weeks ago. At my school, it is required for everyone to take a shower after gym since our classes are spaced so far apart. Nudity isn't completely gone from the locker room, but most of the other students get partially dressed straight from the showers. Some of the guys, however, just come out in their towels but get dressed immediately. Because of a dare, I was going to be the one to take everything one step further.
My best friend has most of the same classes as I do, and we are always daring each other to do dumb things throughout the day, but it's never anything to get us in trouble. This time, though, he said he was giving me the 'dare to end all dares'. He told me that he wanted me to masturbate in front of the whole class in the locker room when we were getting dressed. At first, I told him there was no way I was doing that, but eventually I started to loosen up a little. After he bugged me for about 10 minutes, I finally gave in. I told him that I would do it later this week.
That day during gym, the whole time he was shooting me looks and making jack off motions, and I would just smile and shrug my shoulders at him. When it was time for the class to end, my heart was pounding. While in the shower, I made sure that I was hard before I got out. Erections are fairly common in the shower, but they are largely ignored. Since I was one of the last to walk out, the rest of the class was well on their way to being dressed. My friend looked at me, wondering if I was going to go through with it, and all I did was nod which just made him smile. I sat down on the bench and, knowing I couldn't back out now, opened my towel exposing my rock hard cock to the entire class and slowing started masturbating.
Once one guy saw what I was doing, he nudged someone else to show them, and eventually, the entire class knew that I was sitting there jacking off. I glanced around and saw that just about everyone had stopped what they were doing and were gazing at me in shock. I also noticed that many of them had also gotten hard while watching. Since there were so many eyes on me, I knew I wouldn't last long. When I felt I was about to cum, I leaned back, stretched out my legs, spread my toes and shot an enormous load all over my belly. As I sat there catching my breath, the room was silent except for some of the guys saying "whoa" or "holy shit...". With my hands shaking, I cleaned myself up with my dirty gym shorts, got dressed and walked out like nothing had happened, all the while just about everyone was staring at me in shock still. As I left, my friend, laughing, said he could believe I just did that, and I told him that I couldn't believe it either.
A couple days later, while everyone in the shower, I saw in the corner of my eye one of the shorter freshmen jerking on his long penis. I not only couldn't believe he was doing it, but he was also huge! In the days after that, I noticed more guys jacking off in the shower, including my best friend. A couple guys were even openly masturbating in the changing room while the talked.
I think I might have made masturbating after gym a normal thing!