Sam, “Not everyone agrees that the elbows and knees must be covered…”
Of those that don’t, 50% of their kids text on Shabbos.
If we weren’t so judgmental about the parents then many of these kids would be in frum schools and would not be texting on Shabbos.
Popa beat me to it, but I’ll also respond to the point of the OP.
How many kids really are in public school because the local cheder or Bais Yaakov didn’t accept them? There’s a lot of room between the frummest schools and public school! The parents who don’t dress according to chassidish or yeshivish standards of tznius rarely want to send to these schools. Their kids are in MO schools.
And those who do want to grow and send their kids to a school which has ideals which are frummer than their own, have a choice to make. Either get rid of the TV (for example) or don’t expect to be accepted.
A school has a right (some would say an obligation) to shield the kids from the more sheltered homes from being influenced, through other students, by the very same secular influences they are moser nefesh to keep out of their own homes. (I’m not referring to a case where there are no other frum schools; that’s a different story)
I blame the parents, not the schools.