I'm a Guy and I Can't Seem to Urinate in Public Restrooms

I'm a 16 year-old guy. Considering I'm 16, and in high school, there are multiple occasions where I am forced to use the public bathrooms. The problem is I can't seem to actually use the bathroom. Well, at least when other people are in there. I feel like I should just be able to walk into the restroom, whip it out, and start peeing. But I can't. It will not happen. I almost always have to use a stall, and if someone else is in the restroom at the same time I am; it seems impossible to pee. Then I feel awkward just standing there, because I feel as if the other person is wondering why he can't hear me pee. So I get more tense and awkward, because I feel like I'm getting judged. And then I wonder if they can see my shoes; if they can they'll be able to spot me in the halls and think, "that's the kid who couldn't pee." Please help. I don't know what to do.