A pupil was taught in a box room with the windows blanked out when he was finally allowed back into school after being expelled for violent behaviour.

The teenager - known only as Pupil L - worked under the supervision of a retired teacher and was allowed no contact with other children and break and mealtimes, the House of Lords heard on Monday.

The head teacher took the measures when members of the NASUWT union at the Catholic school in Hertfordshire refused to teach the boy and threatened to take industrial action - even though he had won the right, on appeal, to return to the school.

But Pupil L's lawyer, Cherie Booth QC, is arguing to the Law Lords that he was "denied a proper education".

Ms Booth said the case raised important questions as to what was meant by "reinstatement" and how this process could be affected by the threat of industrial action by teachers.

The law barred "humiliating and degrading treatment" if a head teacher decided against exclusion and settled for a less severe punishment of taking a pupil away from his or her peer group, she said.

As a result of being treated as a "pariah" the teenager, now 17, passed only four subjects at grade A to C instead of the expected seven or eight, Miss Booth told Lords Bingham, Hoffmann, Hobhouse, Scott and Walker.

The school was in breach of its obligation to reinstate Pupil L following the successful appeal by his parents, she added.

Attack in toilet

Pupil L was expelled in January last year over his involvement in an assault on another boy in the toilets.

The victim needed hospital treatment after being punched and kicked by a gang.

An appeals panel found that Pupil L, who admitted that he aimed a kick and missed, was not among those who had directly inflicted the injuries and permanent exclusion was not appropriate.

The teenager was allowed back to school after being excluded for six weeks, only to be "subjected to a regime of isolation from his peers", Miss Booth said.

The Law Lords will also hear of a similar case in London, where another boy - Pupil P - was permanently excluded from his secondary school then allowed back on appeal.

Again, members of the NASUWT refused to take the boy into their lessons.

The case of the two boys comes at a time when Miss Booth's husband, Prime Minister Tony Blair, has pledged to crack down on violent and disruptive pupils.

The hearing is expected to continue for two more days.