Responding to a new report by the Education Policy Institute (EPI) on the impact of head teachers on pupil outcomes, James Bowen, assistant general secretary at school leaders' union NAHT, said: "This report shows that effective school leaders have a direct and positive impact on pupil outcomes.

"Put simply, school leaders make a difference and we should be supporting and investing in them.

"The finding that effectiveness increases with experience shows the importance of keeping head teachers in the profession for as long as possible.

"Sadly, the growing crisis in retention of school leaders means that this is not currently happening with too many leaving the profession prematurely."