Conservatives claimed during a special parliamentary debate last week that heads were being constrained in their drive to push up attainment by the contradictory policy to retain difficult pupils in school. The bad behaviour of some was damaging the education of others and placing extra stress on teachers.

But Peter Peacock, Deputy Children and Education Minister, defended the exclusion targets which lay down “a broad direction” for the way policy should move. “We want to see schools bringing down exclusions. The beginning of being excluded from school is the beginning of being excluded from society. Secondly, it is a focus for attention and drives forward the search for alternatives to excluding young people,” Mr Peacock said.

Targets were not compromising decisions about individual pupils. “In the final analysis it is the headteacher’s judgment of a situation that must carry the day. I would not criticise a headteacher who, having put in place the proper mechanisms to deal with discipline matters, felt, as a final solution, they they had to exclude somebody.

“They must decide whether exclusion is necessary to relieve pressure on the school and to allow time to develop alternative mechanisms and so on.”

Mr Peacock outlined a raft of initiatives, including the alternatives to exclusion scheme, new community schools, personal learning plans, classroom assistants and nursery education, that were helping to cut alienation in the longer term. The ethos and culture of the school were important.

However, David Mundell, deputy Conservative education spokesman, said the 33 per cent target would not work as it bore no relation to what was happening in individual schools. “There is a conflict of interest between local authorities pleasing the Scottish Executive in meeting targets and individual class teachers in an environment that does not meet the issue.”

Mr Mundell praised David Blunkett, the Education Secretary, for introducing learning support units, or sin bins, for disruptive pupils outh of the border.

Nicola Sturgeon, SNP education spokeswoman, also highlighted the contrast, suggesting the English policy was “something we could learn from”.

Mr Blunkett has announced a pound;47 million scheme to establish more than 1,000 units for violent and disruptive pupils who would be excluded for fixed periods. Most will be in secondaries.

Ms Sturgeon said many heads privately admitted they could not exclude because of the targets. Difficult children either remained in the mainstream, continuing to cause problems, or heads used informal exclusions. Resources were not available for alternatives.

Introducing the debate, Brian Monteith, Conservative education spokesman, said 1,388 violent incidents against teachers were recorded in 1998-99 and a further 517 against ancillary staff. Targets could only make that situation worse. “There will be more violence against teachers and pupils and greater disruption to schools with a consequent lowering of morale in the teaching profession. This must affect standards.”

The creation of arbitrary targets would be “prescriptive, self-defeating and damaging”.

Richard Lochhead, SNP, warned that disruptive behaviour was hitting staff morale. In Aberdeenshire, 275 teachers, or 12 per cent of staff, had enquired about voluntary severance and a further 210 were off sick. The council had no units available for alternative education.

Murray Tosh, Conservative, who taught in secondary schools for 25 years, said discipline was teachers’ number one concern. The cause of stress-related illness was the high level of relatively minor misdemeanours. “You cope by trying to be lively and interesting and being non-confrontational. You cope by being persistent, by being patient and deploying your personality,” Mr Tosh said.

Robin Harper, Green Party, a secondary teacher for 37 years, said it was vital schools maintained an atmosphere that supported learning, especially in a deeply divided society with 30 per cent of children living in poverty.

No primary class should have more than 20 pupils, Mr Harper said. “A class of 30 is still too big for a primary teacher to cope with and children do not get the individual attention they deserve.” ATOM