This is a simple one. I was a senior in high school. It was lunchtime. The cafeteria was on the bottom level of the building, so there were big pillars all around the room. Of course, there were a lot of circular tables where students could eat. The bell rings, signaling the end of lunch. I get up and take my tray to the designated tray area. Students are slow to file out, and the cafeteria is still very much full. For some reason, I decide to cut through the tables as I leave, instead of simply walking around them. One table is directly across from a pillar, and the only thing blocking my path between these two obstacles is a chair. I decide to just jump on top of the chair, then jump to the floor and go on my way. So, I jump, but instead of landing perfectly straight on the chair as I'd planned, I land at too much of an angle, push the chair forward with my body weight and land flat on my back in front of half the student body. And EVERYBODY laughed. Some pointed. I tried to laugh along and play it off, but it wasn't happening. That was embarrassing.
Another good school one happened when I was in sixth grade. I was very shy, and pretty much only had one friend until I was in seventh grade, so I was still hesitant to talk to anybody. So, we're in class, and the teacher announces that we should form ourselves into groups. Being unpopular, I decide to just wait until everyone makes their groups and just join the one that's short one or two kids. This time, that group had three or four kids who were considerably more popular than I was. I go to sit at their table, and a popular guy looks at me as I start to sit down and says, "Don't sit at our table. We don't want you." So, I get up and look around for another group to join, but, seeing as I never really talked to anybody, I'd never dealt with that kind of rejection before. It hurts a lot, and I start to cry. It's soft at first and I can hide it, but it gets heavier, and I start shaking/heaving gently and tears are running down my face. The teacher sees me and forces the popular kids to accept me into their group. I'm unable to help with whatever project we're doing because I'm still crying, and the kid who told me off leans toward me and tries to reassure me and get me to stop crying, but it just makes me feel worse. And all this time, I have to try and come to terms with the fact that I just exposed myself as a sensitive dork in front of everyone in my class.
Well, I guess that second one is a little more haunting than humiliating.