In high school (15-17) in certain classes where I could sit in the back and needed to masturbate, I'd undo the top of my pants, unzip, then roll a reservoir tipped condom down my erection to the base pushing back my pubic/shaft hair do it wouldn't get pulled, and do it under the desk, filling that condom. I only got caught once by (fortunately) a male teacher that caught me at the point where I couldn't stop, seeing me fully ejaculating into it. I was afraid he might tell my parents, but didn't.