In a case that will test the right of teachers to prevent unruly pupils returning to mainstream lessons, Cherie Booth QC told the law lords that the boy - now 17 but then 16 - had been denied a proper education to which he was entitled under human rights law.
When he went back to complete study for his eight GCSEs, he was taught in strict isolation by a retired maths teacher in an 8ft by 6ft room, with the glass door panel blanked out.
The boy - who cannot be named for legal reasons but known as Pupil L - is one of two in a legal challenge by parents seeking to overturn earlier rulings involving staff belonging to the second largest teachers' union, the National Association of School Masters/ Union of Women Teachers.
Pupil L was one of six pupils permanently excluded after a fight in the toilets involving around 20 pupils.
The boys were reinstated at their respective schools in Hertfordshire and Lewisham, south-east London, after having their cases heard by independent appeals panels.
The heads ordered their staff to teach the boys, but teachers belonging to the NAS/UWT voted to refuse. The pupils had to be taught in isolation. The union successfully argued in the high court and the court of appeal that the industrial action was legal, because it related to teachers' conditions of employment.
The parents claimed the teachers did not have a valid trade dispute under union law, and the boys were being unlawfully barred from school. Ms Booth told the lords that the case raised "some interesting and topical questions" about "reinstatement" and how that regime could be affected by the threat of industrial action by teachers.
On the first day of what is likely to be a three-day hearing, she said another teachers' union - the Association of Teachers' and Lecturers - reported some 33 separate ballots last year in similar cases where teachers had voted not to teach violent or disruptive pupils. "This is not an isolated incident," Ms Booth said. "There's a pattern emerging of teachers threatening industrial action." Pupil L had not been allowed to mix with other children, she said. The head blocked off the glass window of the door so he could not look out and no other children could look in.
As a result of being treated as "a complete social and educational pariah" - despite his parents having won an appeal for his reinstatement to the school - the boy passed only four subjects at grade A to C, instead of the seven or eight which he had been expected to get, Ms Booth said.
His poor performance meant he had failed to get into the school's sixth form, where he hoped to study science A-levels.
After also failing to obtain any apprenticeships, he eventually left school to take unskilled employment in a factory paint shop.
The case continues.
