In an interview for the Sunday Times Wilshaw said that more than 5,000 headteachers in England are not good enough, based on Ofsted figures showing that leadership is 'satisfactory' or below in 24% of schools. He claimed that the reason why 'thousands' of teachers decide to leave the profession soon after qualifying is because they do not feel supported by the SLT in their school. 

Pupils underachieve in 'coasting' schools where the head is not ambitious enough and makes 'excuses' for low standards, he added.

The full interview with Sir Michael Wilshaw can be found on the Sunday Times website (behind paywall). For a shorter summary see the Daily Telegraph.