I usually have the opposite problem in locker rooms, shrinkage.
I remember one time when I was 12, though. My family was on a beach vacation. We were staying at a state park, and the park had a large indoor shower area that was set up like a locker room. After we were done at the beach, my dad and younger brother and I all went to get cleaned up. There wasn't anyone else in the showers. They went down one side and I went to the other, for more privacy probably due to puberty causing some embarrassment about my genitals.
Anyway, I was naked in a public shower for the first time, although I was alone. There was a curtain, but it lacked full coverage by about six inches. While getting the sand out of my nether regions, I got an erection, and being alone decided to take care of it.
At that point I'd had more than a year of regular masturbation with ejaculation under my belt, so it wasn't difficult to cum quickly. I deposited a huge load in the shower stall, then rinsed it down the drain.
Just then, I noticed two teenagers, probably 14 or 15, had come in my half of the shower room, horseplaying and carrying on. I don't think they were aware of what I had been doing. I walked out of my shower, my erection having subsided but with my cock still plump. Since I had left my bag across the room, I had nothing to cover up with, so i just acted nonchalant, hanging all out. I noticed them sneaking a peek at me as they slipped off their swimsuits and got into showers, and I returned the favor. I toweled off and dressed quickly.