Office for Standards in Education inspectors had concluded that children were "vulnerable" and "at risk" at Periton Mead school in Minehead.
But lawyers acting for the boy only discovered the special school was failing when they searched the Internet.
They found the school had been on special measures for two years - a fact North Somerset Council had not disclosed at a special needs tribunal hearing.
The OFSTED inspection report revealed a catalogue of failings at the school. It said: "The serious problem of pupil control and indiscipline in classes around the site makes pupils vulnerable and puts them at risk."
The 14-year-old boy had gone to the tribunal to win the right to attend the private pound;50,000-a-year Cotswold Chine School because his parents had already decided Periton Mead was unsuitable.
Now the boy is seeking a judicial review. The case will be heard in the High Court on March 10.
Beverley Watkins, the boy's solicitor and an education law specialist with Bristol firm Price Watkins, said the OFSTED report was one of the most damning she had seen.
She said: "The local authority did not disclose the school had any problems and attempted to portray it as a successful and thriving school when clearly that wasn't the case."
The boy has not attended school since November 1997. As well as suffering from Tourette's Syndrome, a neurological disorder, he is dyslexic and has attention deficit disorder.
Ms Watkins said the authority had balked at the size of the fees at Cotswold Chine School.
She called for special needs funding to come directly from a central Government pot instead of the budget of a small authority like North Somerset, which until 1996 was part of Avon County Council.
A spokesman for North Somerset admitted the school's OFSTED report was not disclosed at the tribunal, but said its failings were common knowledge in the area and had been widely reported in the media.
He added: "Whatever decision is made at the High Court, we will be happy to respond to that decision."