How can you prevent unexpected boners?

You can't really prevent them. Random erections are a normal part of puberty and they happen to pretty much everyone with a penis, often for no reason at all.

During puberty, your body is adjusting to new hormone levels, and random erections are a side effect of that. They become less frequent as you get older, but while you're going through puberty, they're going to happen and there's not much you can do to stop them.

What you can do is manage them when they happen. Thinking about something boring or unrelated can help. Flexing a large muscle (like your thigh) can redirect blood flow and sometimes makes it go away faster. Tucking it up into your waistband is a well-known trick. And if you're sitting down, nobody can see it anyway.

If it happens at an awkward moment, try not to panic. It usually goes away within a few minutes. Nobody is paying as much attention to it as you think they are.

You cannot really prevent random erections. They are a normal part of puberty and happen to almost everyone with a penis, often for no reason.

During puberty, your body is getting used to new hormones. Random erections are a side effect. They happen less as you get older.

What you can do is manage them when they happen. Think about something boring. Flex a big muscle like your thigh. Tuck it into your waistband. If you are sitting down, nobody can see it anyway.

It usually goes away in a few minutes. Nobody notices as much as you think.