I've seen several posts about people masturbating in school. I used to do that, and here's my story.
When I was in grade school, I was masturbating all the time at home. I did it by rubbing my crotch on things, mostly my pillow and teddy bear. I liked the feeling so much that one day at school I got thinking about it and had the urge for the feeling (that's what I called it: 'the feeling') and held a book in my lap so that the edge of the book was resting on top of my little boner. If I moved the book up and down it stimulated me, and I discovered a great way to get my feeling. I was lucky in that with a last name beginning with 'S' I sat in the last row (the teacher arranged us alphabetically}, so what I was doing wasn't obvious. I started doing this in 2nd grade. The teacher never seemed to notice that I would 'check out' for a little while nearly every day. I guess other kids daydreamed occasionally, so she was used to seeing kids being unattentive for a while. I was always looking to see if any of the other boys were doing what I was, and all I ever saw was one boy who occassionally would move his hand like he was adjusting himself in his pants, but I couldn't tell if he was actually doing what I was.
As I got older, that habit became less frequent, mostly because teachers assigned random seats, and I couldn't do it without being seen. Whenever I could chose my own seat, I'd chose one in the last row for obvious reasons.
In middle school I started getting horny a lot. This was more than just getting the feeling; this was daydreaming about sex. I'm not gay, but most of my daydreaming was about whether the other boys in my class did the same kinds of things I did. I'd focus on one of the boys who I was attracted to (I had something like crushes on a couple of them at various times) and rub myself to orgasm.
I had a real surprise one day when I ejaculated for the first time. I'd been staring at one of my classmates who I knew got boners in class frequently. I'd see the bulge in his pants, and how he'd nervously adjust himself, and sometimes he'd move his hand, clearly enjoying the feeling he was giving himself. I rubbed, and exploded in my pants, and was surprised to feel a sudden wetness. I knew to expect it at some point, but was caught off guard having it happen in class. It felt good at first, but as it cooled down and soaked my underwear, it began to feel very uncomfortable. Worst, the next class was PE, and I had to change with the other boys without them seeing the mess. I was able to do it, but that experience left me with a conundrum: How to get my feeling without making a mess.
I found a solution. I'd carry a hand towel in my bookbag, and during the day stop in the restroom and go to a stall and install the hand towel inside my underwear. That way when I played with myself I had something to catch and absorb the mess, and at the next opportunity I'd go to the restroom again and take out the hand towel and put it in a plastic bag so I could dry it out later. That technique worked well, except once when a friend came home with me and when I opened my bookbag to get something the plastic bag with the hand towel fell out. He picked it up, and smelled the obvious odor of fresh semen, and remarked, 'so this is how you hide the mess.' That was really embarrassing, since I realized he knew what I had been doing, and I wondered how many other people knew.