Elizabeth West, head of the 400-pupil Charlotte Sharman school in Southwark for 27 years, has accused a small group of parents of mounting a hate campaign against her. Her deputy, Pauline Milton also plans to leave the profession.
Objections from some parents delayed the school's move to grant maintained status from last April to October 1.
Alf Bagnall, chairman of the governors, wrote to parents telling them of a "small minority" who had unleashed a "continued barrage of threats, abuse, intimidations, lies and slanderous allegations against Mrs West". The local police were informed last July of an alleged death threat against the head. The school hired a security guard and installed surveillance cameras at the start of this term.
A council spokesman said they were not told of any threats when Mrs West applied for early retirement last March. As the school was due to become grant-maintained, her application was referred to the Funding Agency for Schools.
Gordon Mott, the director of education, visited the school last week to meet staff, parents and governors. He noted that the school had made its own security arrangements.
Gina Kent, a parent governor, said the governing body was considering legal action to get the small minority of parents to take their children away from the school. Many others had signed a petition urging Mrs West, who has a reputation for running a tight ship, to change her mind about leaving.
John Fowler, assistant education secretary of the Association of Metropolitan Authorities who chaired the school governors in the mid 1980s, recalled that in one year almost all the teaching staff left bar the head. And inspectors of the Inner London Education Authority raised concerns about the quality of teaching, which they reported to the governing body.
Mrs West declined to talk to The TES this week.