Amanda Thain, 54, was sent home on full pay from her £60,000 a year job after a damning report warned that a core group of students had become so “disruptive and dangerous" that others felt unsafe.
A snap Ofsted inspection found there had been a "rapid deterioration" in behaviour within just a few months and that reports of fighting, swearing, smoking and truancy went unchecked.
One girl was thrown out of class more than 80 times, others were sent out 30 times and there was a race-fuelled mass brawl between two gangs.
One large group of girls gathered in the playground only dispersed once a police officer had been called to stand amongst them.
Inspectors found there had been eight expulsions in less than a year at the 924 pupil Levenshulme High School for Girls in Manchester, where the motto is: "More Opportunities to Succeed."
The investigation, which was carried out after a complaint from a parent, came after the school announced plans it was spending £220,000 buying iPads for every pupil to help with homework and "engage" with teachers.
Earlier this year, Mrs Thain had been praised by council bosses by saving the local library from closure by offering to use it for the part of each day as a "'satellite school" for youngsters who need extra teaching – and pay for the upkeep.
However, the school has now been placed in special measures after the sudden surge in bad behaviour led parents to compare it to St Trinian's, the fictional girls' boarding school originally created by cartoonist Ronald Searle in the 1940s which later became the subject of several films.
Ofsted gave it the lowest Grade 4 rating and branded it "inadequate" and "requiring improvement."
One parent said: "We're absolutely astounded at what happened to Levenshulme because until recently it had been doing so well. It had got good grades in previous Ofsted reports and for all intents and purposes looked as if it was improving all the time.
"But something has gone wrong and now it's like something out of the St Trinian's movies. We've heard all sorts of reports of small group girls running wild at the school and it’s impacted on everybody else.
"Some of the youngsters said they didn't even fell safe going there any more. No one knows how it has got to this. Maybe the head has simply taken her eye off the ball."
Ofsted called the unannounced inspection on July 3. Three inspectors spoke to 100 girls and observed lessons and break times.
Its report said: “A core group of students has disengaged from their education and show no respect for teachers or their fellow students.
"They cause widespread disruption to learning and an unsafe learning environment resulting in eight permanent exclusions in the academic year to date.
"Many students feel unsafe in school due to a wide variety of intolerable behaviours including, fighting, congestion and pushing on the stairs, teachers leaving classes unattended, teachers behaving aggressively towards students; and food and drinks being thrown.
"The majority of students behaved well in front of inspectors. Other students were observed at lunchtime, throwing drinks, congregating in a big group, swearing, talking and filming on mobile phones.
Levenshulme High School for Girls in Manchester. (ANTHONY MOSS)
The report said most senior leaders at the school were “unaware” of the scale of the bullying and had "an over generous" view of the quality of teaching due to inaccurate feedback from staff.
Inspectors also warned white British girls were "underachieving" at the school, where pupils are predominantly Pakistani and speak English as a second language.
Mrs Thain, who had been in post for four years, will remain off work pending an investigation by the school governors.
In a recent message to parents she said: "Our school is characterised by high expectations for both achievement and behaviour and our expectations of the quality of teaching in the classroom are as high as our expectations of our students' work."
Patsy Kane, the head teacher at neighbouring Whalley Range High who has been appointed executive head at Levenshulme, said: "A thorough review has been carried out of the school's behaviour and safeguarding policies.
"These have been updated, clarified and strengthened and all staff have been given additional training in safeguarding. Nobody will be in any doubt about the standards of behaviour which are expected from them. They will also know who to speak to if they do have any concerns and can be reassured that these will be dealt with appropriately.
"We are committed to listening to students, having already sought their views, and will be carrying out online surveys on a regular basis to ensure things have improved."
John Edwards, Manchester council's director of education and skills, said: "Swift and decisive measures were taken as soon as the failures came to light.
"We are working with the governing body to ensure that strengthened and robust governance is in place to help the school move forwards."