Pupils at New College, Leicester, took additional lessons four nights a week and went in on Saturdays to make up lost ground, David Kershaw, executive principal, said.
A year ago, only 10 per cent of its pupils achieved five GCSEs A* to C grades. That has risen to 26 per cent this year.
"The success rate is stunning for a school that was in danger of being closed. The pupils, like the staff, have risen to the challenge," Mr Kershaw said.