State schools have to prove that their children are making progress. A school with excellent results can find itself in special measures if they are not tracking progess and all groups of children are making the progress they should. For example a grammar school is expected to do better than the comp.
I feel that private schools should be under the same scunity and have inspections with the same level of rigor as state schools. Parents should have access to value added data to decide if a school is worth the money. It seems to me a bit of a cop out that private schools can be inspected by the independent inspectorate rather than OFSTED.