We maintain freedom of religion for precisely the same reason. Freedom is always at risk, but especially so in times of social turmoil. The wrong way to resolve a crisis is to blindly call for more government control. The right way is to rely on and create voluntaristic social institutions to heal the wounds. Many students wear attire that associates them with a particular social group and conveys a set of specific attitudes.
Athletes frequently wear sports uniforms and letter jackets.
Others wear more striking, more rebellious attire. Now, talk of personality profiles overseen by the police is gaining an audience. A market school has every right to require uniforms if it so desires. A government school has a more difficult time instituting strict dress and behavioral codes, simply because it is tax-funded. Eric Harris and Dylan Klebold are not criminals because they dressed in black.