The kids have a right to a decent education.
IMHO, home-schooling by a bunch of ignorant fanatics exercising parental rights amounts to child abuse. They'll grow up dumb as shit and past a certain point there won't be any remedial education that'll work.
I think it's unfortunate that some parents are good teachers and some aren't, but the state can't afford to get into evaluations which are necessarily subjective to some degree. Best solution is to outlaw all religious, ethnic and home-schooling, and mandate that every kid without exception attend a regular school week in a regular school year at his neighbourhood public school. If the parents want to supplement their kids' education, they can do it at home, after school hours.