Yes it is normal. I can recall having it at age 17 while in my senior year of high school while killing time in the library during lunch hour. During this senior year, I was not masturbating and it likely explains why this would happen with no warning or me triggering it. I suggest you resort to relieving yourself (ejaculating) in private of course, after school hours and it may still occur at anytime, but with far less frequency, likely not at all without your intervention/desire for it to happen.
this only becomes a problem when you are reasonably well endowed & your boners tend to last for at least an hour. sadly. personal experience. i dunno y pple thing that its a good thing. i find it just plain humiliating.
that used to happen to me a few years back all of first period i was hard as shit i had to pretend to be dumb so that the teacher would call me up to the bored or anything cus for that i would need ERECTION DEFLECTION.