Feckless parents of all social classes were condemned by a teachers' leader yesterday for failing to ensure their children arrived at school ready to behave properly and work.

Parents should enter into a "learning contract" with schools to ensure that education standards continued to improve, said National Association of Head Teachers (NAHT) general secretary David Hart.

He stressed the problem was not confined to parents from the lowest social classes, saying "inadequate parenting" was also to be found in affluent families.

Mr Hart outlined his uncompromising message about the damage being done to children's education by poor parenting in his address to the union's annual conference, in Cardiff.

Mr Hart acknowledged home-school contracts could not be enforced by law, but he said they could be used by schools as evidence that they kept to their side of the bargain when parents attempted to blame teachers for their own failings.

He said: "Too many parents collude with truancy and condemn their children to a pattern of crime. Two-thirds of persistent truants have committed a criminal offence."

The Government "cannot be faulted for its attempt to break the vicious link between social and educational under-achievement", Mr Hart told delegates, but its investment in anti-truancy measures was not going to buy parental responsibility.

He said: "Nor do new criminal sanctions necessarily change the mindset of those who commit or condone anti-social behaviour.

"It is the attitude of these families, who know all about their rights but precious little about their responsibilities, that has to change."

Schools needed support enforcing discipline and they should be able to punish pupils with detentions.

He said: "Parental support for behaviour policies is crucial.

"This means not spuriously asserting that sanctions attack the civil rights of their children. This means not defending the indefensible."

In an attack on some parents' fecklessness and irresponsibility, Mr Hart said: "School staff are not surrogate parents, they are not social workers.

"They should not be expected to waste valuable teaching time doing the job that should have been done before the child sets foot in the school."

He said: "Parents can be a profound force for good, and a substantial majority are, but the problems faced by schools are steadily increasing. So politicians of all parties need to recognise the bigger picture.

"Government cannot control how families behave, but it can understand the context in which schools operate and support their staff in their struggle to raise standards, too often against the odds.

"Teachers continue to be blamed, unfairly and illogically, for under-achievement that is so often home-based."

Margaret Morrissey, spokeswoman for the National Confederation of Parent Teacher Associations, said: "These comments do nothing to promote home-school partnerships and to build on the very good working relationships between parents and teachers in many schools across England, Wales and Northern Ireland.

"We would like to see the focus moving to celebrating the increase in information provided by schools and Government, which is enabling parents to work with teachers and support pupils through the entirety of their education."