Students should wear gym clothing uniforms because it helps move around &affordable. If we were to buy our own gym clothing from other stores, it's more pricey and not to mention there are people who can't afford that. Therefore, schools sometimes give a pair of gym clothing for free (clean of course). After purchasing gym clothing, it's easier for students to run or such, and athletes should already be performing great since they're such star quality in physical fitness; gym uniforms is not a strong force field that disables them from being athletes in the first place. That's just complaining. If its so restricting, it's just depends on how the school has their gym uniforms designed.
So do I keep going?
Well another reason to wear them is safety. Some schools have PE outside campus so, obviously the instructor could identify the missing student to others by indicating it's clothing uniform. Hence, why gym uniform should be allowed.