I'm sorry this situation is feeling so awkward for you. If it helps, you're far from the first person with a penis to experience this situation (and you're probably not the only person in that locker room dealing with this issue from time to time). When you're younger, you often get erections for no obvious reason, or from stimulation that's not even remotely sexual. Your best bet is probably to ignore it when it happens, which will help it go away. Does that all help?
I'm sorry this situation is feeling so awkward for you. If it helps, you're far from the first person with a penis to experience this situation (and you're probably not the only person in that locker room dealing with this issue from time to time). When you're younger, you often get erections for no obvious reason, or from stimulation that's not even remotely sexual. Your best bet is probably to ignore it when it happens, which will help it go away. Does that all help?