Keith Hargrave, executive head of the Ramsgate school, Kent, was brought in to run the secondary modern in the summer after inspectors identified 44 per cent of teaching as unsatisfactory and placed it in special measures.
He angered unions by threatening to place 24 of his 34 teachers on "fast-track capability proceedings", which would give them just four weeks to improve or face dismissal. Since then five have resigned and left, one is leaving at half-term and three are leaving at Christmas. Another three are off sick.
Mr Hargrave said six youth workers had been brought in to cover the resulting staff shortages, two of whom were not qualified as teachers.
The school also had another five unqualified staff working as teachers. But this was still better than when he arrived when there were around 15 and they made up 40 per cent of the staff, said Mr Hargrave.
Allan Craig, Kent's national executive member of the National Association of Schoolmasters Union of Women Teachers, said: "If you have got youth workers taking classes, some of whom are unqualified, then what chance will that give the kids? I am concerned that they will not be up to the needs of the curriculum."
Mr Hargrave said the unqualified staff were under supervision and were only temporary. He expected to have appointed permanent, qualified replacements within two weeks.
The other 12 teachers originally threatened with the sack were having their futures decided during interviews this week. Mr Hargrave said that of the first six interviewed, none had been put on the fast track to dismissal.
Two were facing capability proceedings but had been given an extra four weeks to prove their competence.
"Obviously, this is a difficult time for everybody, the students as well as the staff," he said. "But once we have got through this period we will move forward."
Dick Boland, National Union of Teachers' south-east regional secretary, said he thought the school was acting in a more professional manner than it had been.