If you're in the early stages of puberty, what you're experiencing wouldn't be all that uncommon and is normal, that is, to have them frequently. However, what are some of the things that you're doing that are the most likely things to cause the erections, please?
For instance, sitting in a classroom and thinking about a girl or girls that you like; having your pet lie down on your lap; lying on your stomach in bed, being close physically to someone to whom you're attracted, etc.
When you find yourself in this position,think of something awful.Something that really turns you off.
Or do a calculus problem in your head.
Recite the alphabet backwards.
Use your books to hide it during school.
You could try wearing two pairs of boxers.
It is not uncommon.
Yes, it's not uncommon, and I was going to get into the "thoughts" thing. But, thank you for pointing it out!
As a former school music, choir and band teacher for a number of different grades, I always was careful for mixed choruses as to when to ask everyone to stand up!