Education ministers are aiming to reduce expulsions - "permanent exclusions" - by a third within the next four years, and have set a similar target for reducing truancy levels.

Amid concerns that some schools are expelling difficult pupils to improve their exam pass rates and league table performances, the government proposes that permanently excluded children should continue to be counted as a member of the school for the purpose of the tables.

Such a move would automatically depress the league table standing of a school which expels a high number of pupils, as permanently excluded children usually complete their education with few or no qualifications.

David Hart: "The government's approach is misconceived"

But in its formal response to the government's proposals, the National Association of Head Teachers attacks the notion that schools are expelling poor performers for league table purposes.

"Heads have to justify their exclusion decisions to governing bodies and often to appeal panels," says the document. "The suggestion that those decisions are not taken in the interests of all concerned, including those of the excluded pupil, is offensive and unwarranted.

"This is clearly a deliberately penal attempt by the government to reduce exclusion rates by artifical means. It is a very poor substitute for a coherent strategy to reduce exclusion in a manner which takes account of the interests of the entire school community."

The association also criticises the decision not to include independent schools in the proposed new system, describing it as "unjust".

Its General Secretary, David Hart, said: "Nobody in their right mind could ignore the need to reduce exclusions and truancy. But heavy-handed attempts to cut exclusions by forcible means, in pursuit of artificially fixed national reduction targets, will not wash.

"Schools can hardly be expected to respond positively to the government's agenda when it has outlined no overall strategy to support exclusion and truancy reductions."