Students are minors, and the admins are charged with education and protection, not the promulgation of political discourse to satisfy the restlessness of bored libertarians. Put 'em in uniforms, and teach 'em to read, write, and calculate. If a t-shirt offers distraction, make the kid take it off, turn it wrong side out, put if back on, go home at the end of the day and
do not wear it again to school. If you allow enough Muslims into the US in full hijab--they'll be assaulting women on the streets who do not wear hijab. Turkey understands this--even the French are figuring it out.
I repeat--the hijab is a gangland marker--as surely as grafitti sprayed on the walls.