Mario Citro, acting head of The Ramsgate school, Kent, gave the bad news to 25 of his 34 teachers in interviews at the end of last term.
The secondary modern, where only 4 per cent of pupils got five Cs or better at GCSE in 2002, was put in special measures earlier this year after inspectors said 44 per cent of the teaching was unsatisfactory.
The decision to place the staff on "fast-track capability proceedings" was taken by Mr Citro and his executive head, Keith Hargrave, after monitoring them for several weeks, and was approved by the local education authority.
The pair were seconded to run the school after the previous head, Nick Hunt, resigned after just two-and-a-half terms in May.
Angry teaching unions said they have not ruled out industrial action and are demanding a meeting with school managers.
Dick Boland, National Union of Teachers south-east regional secretary, said the move was "bizarre in the extreme" and had left staff devastated and demoralised.
He said that such procedures should only be used where staff were "irredeemably incompetent" and he could not believe that was the case for so many teachers at a single school.
But Mr Citro said: "The evidence would suggest that at the end of last term that is what they were. I don't think we have gone over the top. In my judgment what the children get here is far below what they have a right to receive."
Education consultants from a private firm, Mouchel, will help to assess whether staff are improving.
Mr Citro said he feared this year's GCSE grades would not show any significant improvement.
Such results put the school firmly on the Government's hit-list of underperforming schools.
It is due to be replaced by Marlowe Academy, a city academy sponsored by Saga Holidays and specialising in business and the performing arts, in 200506.