I'm not sure what concern it is of yours, Concerned Citizen ETown.  If the parents and students are ok with missing a short period  of class to fit in Friday prayers, seems to me that's their business.  Refusing it would likely result in parents removing children from the public school system with resulting lost opportunities to mix with non-muslim children.  I'd say that courts would almost certainly think that's the case, but I'm sure you won't accept that.