The Ofsted report into Woodlands School in Aspley, Nottingham, also says the school was failing to give its pupils an "acceptable standard of education."
The report also says the school's ability to improve was "uncertain" because some senior staff did not understand safeguarding and positive behaviour approaches well enough and did not demonstrate high enough expectations of pupils' progress.
However, they did note that ways of improving teaching led by the executive headteacher had been successful so that no teaching was inadequate.
Teachers at a Nottingham special school locked pupils in classrooms to stop them running out of classes, according to a report by Ofsted.
Woodlands School, in Beechdale Road, Aspley, Nottingham, was put into special measures following an inspection in June.
A report published by Ofsted last week said the school, which has 56 pupils aged 3-16, did not adequately safeguard pupils because some classroom doors were "locked inappropriately" when the children were in the room to stop them from running out.
Inspectors said the practice had been condoned by on-site leaders and had not been picked up by the governing body or the executive headteacher.