A few thoughts. Like it or not, holding proms is not a primary or even core function of schools. If a school board, for whatever reason, wishes to host a prom at a school - so be it. Similarly, if they don't, so be it.
If you challenge the way an organizer or host does things, or the rules they set, they are well within their rights to simply fold their tent.
Activism is not risk-free. Your opponents may well decide that rather than dealing with you and/or your issues they would rather fold their tent and go home - as is their right. If they do, you are left with either doing without or throwing your own party.
Suck it up, and start planning a better party without the petty bureaucracy and micromanagement imposed by the average school board.
That the Itawamba County school board chose to act in a petty and bigoted fashion is without doubt, and that their choice of response was reactionary at best (likely subjecting the girls in question to serious hostility from their classmates) and reprehensible. But simply being vile is not a legal matter, per se.
Again - throw a prom, a better one that's bigotry-free and that allows students a maximum of freedom while providing the structured environment necessary to ensure student safety and a minimum of negative consequences (i.e., steps should likely be taken to keep vodka out of the punch bowl and post-prom pregnancies to a minimum).
Correction: Mississippi, not Missouri