How do you prevent having an erection in pe class?

Distraction.: Erections can be embarrassing in social situations but are a normal physical response to sexual arousal and stimulation. The best way to prevent unwanted erections is to distract yourself from these stimuli. Any erections that result from physical injury or trauma in pe class is, of course, and emergency.

Work your legs: The simplest answer is to avoid sexy thoughts (impossible for most teenage boys - men too!) however if you are stuck in a potentially embarrassing situation and can't excuse yourself - work the large muscles of the legs (this will shift blood flow to them and tends to reduce blood flow getting trapped in the penis - how erections are made). As example "wall sitting" is effective - see diagram.

Hide it: A teen or young adult man can't prevent erections. Just feeling the power of your body and the thrill of being with other young people is going to turn you on -- as it well should. Learn to adjust yourself quickly -- in the midline, pointed up so it's less noticeable. If anyone does notice, so what? They'll understand. Enjoy being young.