It blames the rise on sloppy parenting and on the Government’s decision to abolish “indefinite” exclusions. Schools are turning to “permanent” exclusions instead.

The union has produced an analysis showing that 22 out of 29 authorities have experienced what it describes as “dramatic” rises in the number of children permanently excluded in the first three months of term. In two cases, Durham and West Glamorgan, the figures already equal all of last year.

Under previous legislation, schools were allowed to exclude pupils for an unspecified length of time - letting them return only if agreed conditions were met. This was abolished under the 1993 Education Act however because the Government believed that in practice pupils were left in limbo.

But the NAHT claims its members have lost an important sanction. “Indefinite exclusions often were used constructively to create time to diffuse situations,” it says in a letter sent this week to Education Secretary Gillian Shephard.

Heads now have a choice between fixed-term exclusions of up to 15 days, or expulsion. “If it were the Government’s policy to keep the rate of permanent exclusions down then it appears signally to have failed,” says the union.

“The Government should restore the indefinite exclusion sanction as quickly as possible,” said the union’s general secretary David Hart this week. “Many parents have abrogated their responsibilities and their children are out of control. Heads and staff are sick of being held responsible for the consequences of what is clearly a breakdown of discipline in society.”