Inspectors were forced to break up fights between rowdy pupils during a visit to a failing secondary.
Members of the Ofsted team had to intervene three times while observing lessons at Jeff Joseph Sale More technology college, in May, to prevent injuries to pupils after staff lost control.
Behaviour was so bad that teachers at the 1,200-pupil school had an internal helpline to call for assistance from colleagues when violence broke out. However, help did not usually come quickly enough.
"In the worst cases the teacher has no control over the class at all and highly dangerous situations arise," the Ofsted report said.
"On three occasions during the inspection, an inspector had to intervene to prevent students being harmed during lessons."
The school, in Sale, Cheshire, was put into special measures after the scathing inspection report.
Inspectors blamed poor behaviour on boring, inappropriate lessons. Some were so easy pupils were asked to colour in pictures, copy from the board or do word searches, they said.
They found pupils were unhappy with their school and worried about bullying and bad behaviour. But they lacked self-control and depended on the efforts of teachers to maintain order.
Bullying, including name-calling, was "part and parcel of the general unconcern for each other's feelings and well-being".
The quality of teaching was described as poor in most lessons, with low expectations of pupils by staff and "haphazard" homework.
However, standards in foreign languages and health and social care were good, with little bad behaviour. Staffing was stable and lessons were pitched to the ability of pupils.
Overall, 36 per cent of pupils gained five or more grades A* to C at GCSE last year.
Kathy Leaver, who has been headteacher at Jeff Joseph for a year, said:
"Everyone involved with the school recognises the seriousness of this situation but parents should know that we are working together to overcome the issues and improve things for all our pupils.
"We will be building on the successes identified in the report and continuing to introduce robust policies that will help focus our attention and turn things around."
Chris Pratt, executive director of Children and Young People's Services at Trafford council, said: "With the commitment and determination of the headteacher, staff, pupils and parents, I am confident that major improvements will be made quickly."
The school's website states that the secondary has been oversubscribed for years, and it has a happy environment with strong pastoral care.
John Docherty, vice-chairman of the parents' association, whose son Jonathan, 14, is a pupil, said: "The report came as quite a shock to everyone but it will give the management the leverage they need to turn things around.
"I don't know how you deal with bad behaviour. You can't lock the pupils up but perhaps we need to exclude the trouble-makers.
"I have never considered removing my child from the school. However, it appears there has been a gradual deterioration of standards over the years."