Me too, under the table in certain classes. I always had a reservoir-tip condom I'd roll down my erection first before going all the way with my right hand. I dud my best to disguise my pleasure, but got caught once by a male teacher right as I started ejaculating. He saw me take it off, tie it at the top then wipe my dick. I was afraid he'd call or write my parents, but didn't. I figured because I used the condom and didn't make a mess.