Students attempted to break through locked doors and set off a fire alarm during an Ofsted visit, according to the watchdog's report.
Teachers at New Leaf Inclusion Centre in Walsall told inspectors they were scared for their own safety, and the report found the school was dirty.
Ofsted rated the pupil referral unit inadequate - the lowest possible rating.
Walsall Council said the inspectors' findings were "hugely disappointing".
The school was rated inadequate across all areas by the watchdog after what it described as a "rapid decline" at the school.
What does an inadequate rating mean?
- This means Ofsted judges the school either to have serious weaknesses or to require special measures
The report described pupils' conduct as "often very poor".
"Until very recently staff have been concerned for their own safety while working at the school. Some parents refuse to send their children to school because of safety concerns," the report said.
Ofsted also said buildings were "barely fit for purpose" and pupils gained "no obvious benefit" from attending the school.
Council failed to intervene
Inspectors said Walsall Council had "failed to intervene" adequately at the 142-pupil school for children between five and 16, many of whom have been excluded from previous schools.
Walsall Council said they were "hugely disappointed".
Portfolio holder for Education and Skills councillor Chris Towe said: "Since the inspection we have been working with the headship of New Leaf to ensure all of the issues raised by Ofsted are dealt with."
What is a pupil referral unit?
- Pupil referral units are short stay centres for pupils who are educated other than at mainstream schools or special schools
- They admit pupils with behavioural difficulties and others who can be identified as vulnerable