Part and parcel of the fact that public schools generally do a horrible job of teaching anything useful or necessary. My wife and I have spent the summer bringing our son up to speed with all the things he should have learned last year in the third grade, none of which the school did properly. On the other hand, he can quite confidently lecture you about the evils of smoking, so it's pretty clear that they can teach the things that they choose to. Broken by design, the public schools are...
I had to drive my kids to the publc schools of our choice; two kids times twelve years. I don't kow or care whether it was strictly legal, but the schools never complained because they are funded according to enrollment and attendance.