Kelly I would argue it to your principal (I love debating haha) but if she is focusing on you wearing bodyliners and covering the shell for school then maybe it DOES violates a dress code rule somewhere. Ask what it is but try on all your uniforms before and check carefully-are your shells too small or too short, does your stomach ever show, can you feel any bare skin showing when sitting- reach around and check, do girls have tattoos or piercings showing, are any bra straps showing, deep necklines etc. If yours is okay, make sure you check out your teammates to see if they have anything that could be dress code related. If everything is legit than you should argue-but if not than get those shell tops altered or cover up with liners underneath. And check your skirts as well, make sure they are proper length or wear leggings just in case.
Just my advice, let us know what happens.