Anyone can go to the courthouse, file some papers titled something along the lines of "Complaint for Injunction", pay a fee and "sue" the IHSA. That's day one of law school - anyone can file a lawsuit.
You then likely get a response from the IHSA, with the caption: "Motion to Dismiss Complaint", which in this case will likely be about three pages long. In those three pages, the IHSA will set forth that it is a voluntary association of school districts, that the membership voted on this proposal, and it passed, and that the person who filed the lawsuit has no standing to contest the decision of the membership. The concept of standing will take a month or more of your first semester of Civil Procedure, and after that month you still will not have the foggiest idea what standing really means. In this case, however, basically the failure of the Petitioner/Complainant to be a member of the IHSA will likely be the focal point of the argument.
The relief sought in the motion will be the dismissal of the lawsuit for lack of standing, court costs, and attorney's fees for the filing of a frivolous lawsuit. I would expect that the Motion will be granted, and the Plaintiff/Complainant will be ordered to pay the IHSA's attorneys fees.
A more likely approach to this would be for a group of startled schools to propose a new question to the membership asking for the elimination of the district concept. Other schools might propose some different computation of the districts or classes. Such questions could then be put to vote of the membership again...