To be clear, it is always unintentional. You cannot control wether or not you'll have one or in a very limited manner.

When aroused, yes but it's not like you control or not to be aroused. You just are from whatever is happening in front of you.

You can intentionally trigger it by watching something arousing, rubbing or whatever, but it is still a trigger. You do not "control it" in the same way you control your arm or eyes mouvement.

So unless someone is purposely trying to have an erection, a random erection can occur and is never controller. Especially for a teenager... The amount of times I had a random erection for virtually no reason. Like... The friction from my short after standing up from class was enough.