Well what's this physical for? I've had to take yearly physicals for soccer for a while now, and I know that I don't have to take a piss test. Plus even when I went in for a check-up around 18, I didn't have to piss. But still, find out for your specific situation.
Try and find out exactly what you have to do, and furthermore, will it matter if you test positive if it's not for sports and just a check-up? I mean, it's not like you're going to flunk life with a positive test.