Students at Private Schools in the UK Gain Two Years on Students in State Schools

This article dismisses without comment the notion that the difference in results between private and state schools in the UK can be entirely due to teaching quality? But why not? It is hard to see what else it could be, since the study seems to control for everything else. The private advantage, I submit, is that teache4s in private schools do not need teacher certification, and they are dismissed if the parents are not happy with their performance.