First, you need to see if your school offers some sort of remedial spelling class, no offense, but you can't go through life spelling like a hillbilly and expect people to take you seriously.

Second, I'm not a doctor but I do have a 10 and 12yr old boys at home who have asked the same kinds of questions, so my answer for you is based on experiences with them. The short answer: stop worrying about it, nobody wants to judge you at 15 for what your sexual parts look like or what you are doing with them except maybe your classmates.

As for your physical - they see kids ages 10 - 18 for those, so they see penises probably ranging in size from 1" - 10", corked, tilted, whatever. When they go to grab your scrotum, that's all they are doing, they aren't "checking you out" and if they are, they will probably end up in prison before too long. Your physical is a process to ensure that you are healthy enough and are progressing through your adolescence enough to participate in rigorous physical/sports activities.

As for maturbation - if you produce semen and can ejaculate then yes they could take a urine sample from you, and detect if your protein levels were a bit high. They of course would have to take this sample and run the test within XX number of hours since the last time you masturbated, if you maturbate the night before and go in for your physical after school at say 4pm the next day, they aren't going to be able to tell that you masturbated, if for some REALLY very strange reason they decided to scan your urine.

Last - everyone ESPECIALLY in your age group masturbates. Your body does some pretty funny things with the pheromones and hormones other chemicals in your brain and you are merely satisfying an INSTINCTIVE urge that comes with adolesence. I guarantee you'll foster lots of respect if you are simply just masturbating, and not banging every girl (or guy) in your school.

Cheers!