Sheffield Council has apologised for failing a child with special needs who was left without the right support for over a year.
An investigation by the Local Government and Social Care Ombudsman found delays in the authority's assessment of a city teenager, who has not been named.
There were delays agreeing and then putting in place the special educational needs support he was entitled to - and that limited the number of GCSEs the boy took.
The case also uncovered similar incidents across the city.
The council has already decided to review its procedures, and agreed to pay the boy's mother £1,500 to use for her son’s education and £300 to acknowledge the frustration, time, trouble and uncertainty caused.
Last month the authority admitted making mistakes. City charity Sparkle Sheffield organised a march through the city in protest at the council's failings.
Founder Liesje Dusauzay said the ombudsman's findings echoed what hundreds of parents already knew.
"Sparkle Sheffield are aware from our own direct experience of children being kept out of school far in excess of 18 months.
"In some instances the council is not meeting its SEND code of practice duties and responsibilities and as a result forcing children out of their essential education.
"Children and young people who are disabled are continuing to greatly suffer, despite the promises that things would change in Sheffield, and equalities and human rights are continuing to advocate for these very vulnerable children.
"We are continuing to call upon the council to make redress and to support all children into the education they are entitled to."
All councils must complete an education, health and care plan within 20 weeks when they accept a child that needs support.
In this case, the ombudsman found Sheffield Council was six months late completing the plan.
It issued an earlier incorrect version, which added to the delay and required the mother to attend mediation sessions.
The authority then did not resolve a disagreement with the boy's school about funding the support, so his educational provision was not put in place for a further 12 months.
The boy decided to reduce the number of GCSEs he studied based on what he thought he could achieve without the right support.
Michael King of the Local Government and Social Care Ombudsman said: “Long delays are an overriding feature of the complaints we investigate about councils providing SEN support. And as this case shows, it can potentially leave children failing to reach their full potential.
“All children are entitled to an education that meets their needs and helps them achieve their aspirations – it should not matter whether a child has different needs. But in this case the family had to battle just to get the support the boy was entitled to.
“While it’s difficult to make up for the missed support at such a critical point in the boy’s schooling, Sheffield Council has agreed to act to try to put things right for the family. I also call on all councils to reflect on the learning from this case, and consider if it could help inform service improvements in their area.”