Two teachers and a member of the support staff at Windlestone Hall School in Rushyford, near Durham, were suspended pending the investigation's outcome.
Complaints about the school, whose 101 pupils include many with severe behavioural problems, are thought to go back two years. In July this year, police and social services decided to review previous complaints made about members of the school's staff after a boy's arm was fractured.
Russell Lee, principal education and welfare officer for Durham County Council, which runs the school, said the unusual step of closing the school had been taken because police and social services felt this would help their inquiries.
All the pupils, who are aged between 10 and 16, are being interviewed by members of the joint police and social services unit and their parents have been told.
There are thought to have been about a dozen complaints against members of staff over the past two years.
Alan Miller, Acting Assistant Chief Constable of Durham Police, said police had investigated previous complaints and had sent some files to the Crown Prosecution Service but no action had been taken.
He added: "Teams of police and social services investigators have spoken over the last weekend to pupils who attend the school and as a result of what they have been told further inquiries are clearly necessary.
"We will be re-interviewing a number of children in greater depth as a result of what we have been told."
It is understood that in July this year, a pupil sustained a fractured arm and police and social services decided to review previous complaints made against members of staff at the school.
Mr Lee said that two teachers and a member of the support staff had been suspended.
He said the school introduced a policy in January called Team Teach, which aims to use physical restraint to control pupils only as a last resort.
The investigation team is also examining allegations that children were locked in solitary confinement as a form of punishment.
He added: "You have to understand that many pupils at the school have severe behavioural problems and this is not an easy place to work in."
The council would try to open the school again as soon as possible, he promised. "We want to restore a measure of normality for pupils."
Officials are investigating the possibility of teaching the children in their homes until the school, which has been operating for more than 20 years, reopens.Reuse content