Education Secretary Nicky Morgan: 'This is a period of transition'
The number of under-performing state secondary schools has more than doubled in a year.
Leagues tables show 330 schools failed to reach 40% of pupils attaining five good GCSEs including Maths and English.
Ministers have toughened exams and banned re-sits and some vocational qualifications from performance tables.
The Education Secretary, Nicky Morgan, has defended the changes.