Sir Dexter Hutt metes out short, sharp shocks to badly-behaved pupils, sending them to an isolation unit for up to three days where meals are delivered and toilet breaks supervised.

Separated from their friends until the end of the school day, the pupils are given no teaching and made to fill in worksheets under the watchful eye of school staff.

This week Sir Dexter, executive head of a federation of secondary schools and one of Labour’s favourite head, defended the policy following criticism from the National Union of Teachers.

“For some students who are misbehaving, social interaction is more important than work. If they are socially isolated they miss that outlet,” he said.

Sir Dexter, who was knighted in 2004, is listed in Who’s Who. Ninestiles - the Birmingham school he has run for 17 years - has been visited by Tony Blair, the former school standards minister David Miliband, and a succession of civil servants.

Today he is responsible for two other Birmingham secondaries, the International school and Waverley, catering for more than 3,500 pupils.

Under Sir Dexter’s “Behaviour for Learning” programme, badly-behaved pupils can be sent into an isolation unit for up to three days. A Birmingham NUT representative said this week that the unit was draconian and that some pupils sought time in it as a badge of honour.

“With a lot of students it escalates the problem. All the other wannabe bad boys or girls want to follow them.”

But in an interview with The TES, Sir Dexter said that his methods were hugely effective. Two years ago pupils were said to have regularly threatened teachers at the International school and to have thrown objects at them. In one incident, a teacher narrowly escaped having her hair set on fire.

The NUT even called for a risk assessment to ensure one class met national Health and Safety Executive regulations.

“Behaviour was appalling,” said Sir Dexter. “It was as close as you could get to students, a minority anyway, holding overall sway within the school.”

So he had no hesitation in imposing his well-tested discipline policy, already in place at Ninestiles and Waverley. It is based on a series of “consequences” which all staff and pupils have been told about.

A misbehaving pupil gets a verbal warning (consequence one), a second verbal warning, detention for one hour and then a spell in the isolation unit, if behaviour does not improve. Any pupil emerging from isolation - the unit can take up to six at a time, sitting in blocked-off workstations - can still fall foul of the fifth consequence if they continue to misbehave: exclusion.

Sir Dexter, who has seen the proportion of good GCSE results almost treble in two years at the International school, has not been afraid to use this sanction there. Last year 444 pupils were given fixed-term exclusions and 17 were permanently barred.

“The International school was the most improved in Birmingham last year. It has been through a period of high exclusions - most fixed-term, we hardly ever permanently exclude - but after making the point the number has decreased,” he said.

The Office for Standards in Education, which gave the school a glowing report following an inspection before Christmas, said there had been criticism from a minority of pupils about “inconsistency” in the way the policy was administered, but said it had successfully improved behaviour in a short time.

* graeme.paton@tes.co.uk