A council accused of failing disabled children in order to save money has admitted it has made mistakes.
Campaigners said children with autism and other special educational needs (SEN) are suffering from a lack of support in the classroom.
Children's charity Sparkle said Sheffield City Council is obstructing parents' efforts to find suitable schools for their children.
The council said it "truly regrets" its mistakes.
SEN reforms in 2014 saw the introduction of education, health and care (EHC) plans which are legally-binding agreements local authorities must abide by, but which cost them money.
Sparkle, which supports children with autism and their families, said by delaying EHC plans, children are are not being allocated schools in time or are sent to educational settings which do not meet their needs.
The group, which has been running a campaign called "Stop The Abuse", said this causes stress and anxiety, sometimes leads to self-harm and is in effect "abuse".
Liesje Dusauzay from Sparkle said: "Our children are being left to suffer extreme distress in unsuitable environments without the support they desperately need.
"This is destroying children's lives and their families lives."
Sheffield City Council said in a statement: "We are continuing to work hard to bring about improvements in assessing, supporting and implementing the right provision for children.
"We recognise we have not got this right in the past, and despite improvements we still do not always get it right currently. We truly regret this."
But solicitor Hayley Mason, who advises parents taking legal action against local authorities, claimed councils around the country are withholding information from parents in order to save money.
She added: "They can save their sorries for themselves quite frankly, because it does parents no good."