That's not exactly true. First of all, no one has given an exact final say on whether Constance was invited to the parent-sponsored prom, or if it was okay for her to bring her gf or wear the tux. Second, there is another prom set up that they can go to not far away, that is being sponsored by a gay-rights organization. She can most definitely go to that one, with her gf, and wear a tux. So, although she won't be able to go to the canceled prom, just like all the other students, she would still be able to go to at least one, if not both, of the private proms.
Also, if this high school ever does plan to have a school-sponsored prom again, or pretty much any school sponsored event, they will not be able to discriminate against homosexual students. And, depending on the outcome of the case, they will have to be very careful about their dress codes.