This was a "waste of talent". Parents and government "rightly expect better".
A head teachers' leader said the government had changed the target and it was wrong to say all those schools were giving children a bad education.
Lord Adonis told the Headmasters' and Headmistresses' Conference that standards in the state sector had improved significantly in the past 10 years.
"But it is still the case that in some 800 secondary schools, fewer than 30% of 16-year-olds achieve five or more good GCSEs including English and maths.
"The waste of talent and potential this represents simply isn't acceptable for the future.”
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