ChipAyten,
A trace of protein in the urine can be caused by many different reasons. In young, active people, protein is more than likely the results of exercise. If there is more than a trace of protein, there may be something else going on, or you could be one of the few that always spill a bit of protein.
From a DoDMERB standpoint, if you have a trace of protein on your physical exam, DoDMERB will request 2 repeat urines done 3 days apart, while refraining for strenuous physical activities.
If those 2 repeat urines show any protein, even a trace, it will be a disqualification for proteinuria. Waivers for proteinuria are not that frequent.
My advise to you, and to all parents and applicants, if you have not had the physical exam done yet, 3 days prior start refraining from strenuous physical activities and drink LOTS of water. Its not to say don't do anything for 3 days, but do take it easy and don't push yourself. If you are involved in school sports, talk with your coach and explain that you need to take a couple days easy. Do this and you should not have a problem with the urine test.