If the parents are not likely to expose their kids to the things which I don’t want to have my kids, in turn, exposed to, then I’m not advocating exclusion. If they are, then it’s their own choices which are “not giving their kids a fighting chance”.
I understand where you are coming from. However, it is not our “daas” that should decide the risk/reward calculation of accepting (or rejecting) a child whose parents “might” (or even “do”) expose their children to something that we may agree we do not want our children exposed to. That is why a Rov who knows the school, family, child & situation should be asked in every individual case.
Now of course, the school does not have to accept all comers, and there are good reasons to not accept a child (for example, the child is not from the area and there is no room, or the school can not afford to take the child that will not pay tuition over the child that will). The possibility of “exposure” to “something” (and it may be as innocent as an uncle or sibling who is not frum or is frum but (Chas V’Shalom) wears a blue shirt) is sometimes very much out of hand, and should be an individual shailah.