There is no real "that level"; average are misleading without better context. Bill Gates walks into a bar and "the average net worth is 1 billion per person".

> At that level public schools shouldn’t exist as private do a better job by all metrics.

Private schools have the liberty to pick and choose their students. A special needs student that requires extensive care throughout the day will be rejected by a private school, but the public schools have to accommodate the student. Most private enterprises are efficient precisely because they select their customers with a view to minimum cost and maximum revenue. This is fundamentally incompatible with a public teaching system that has to serve everyone.

Not much, if at all, when you correct for student body makeup. Private schools do a good job of selecting the students that would do better anyway.