If a school does not listen to the VAAD then the whole entire town should know about it because the Rosh of that school should not be allowed to participate in any Vaad or any community conference or council.
There are ways of handling a mix of kids and that is to divide the classes accordingly. Or to open another class in the appropriate school. It is up to the Vaad as a group of frum and reliable individuals to figure out what is best for the students that were left out.