OP, here's what you do.
Get a bunch of you're friends and yourself, wear respectable but out of dress code clothes, and stand in a hallway. Tell the administrators that you are practicing your constitutional right to gather and protest against injustice and the surpression of free speech. And then don't move. They'll threaten to kick you out, threaten everything, but don't move.
This only works at a public school, btw. If you're going to a private school, well then it's your choice to go there so...