Council officials too would feel the full weight of the law where schools force teachers to cover for more than 38 hours a year or to do one of more than 20 “banned” tasks.

The demand for tough action comes from Britain’s second biggest teaching union, as it talks tough on workforce reform.

The NASUWT wants other unions to back its calls for jail sentences and fines for schools that deny teachers their legal rights to better working conditions.

The Government has already said teachers can take employers to tribunals if they flout the workforce deal, agreed two years ago.

But Chris Keates, general secretary of the NASUWT, said: “We want to send a strong message that the law is the law and people must abide by it.”

The union calls for action in a motion to the Trades Union Congress in Brighton next month.

Latest figures show that all but 10 primary schools in England have implemented the deal, at least in part. But the union fears some may try to “whittle it away” in September and wants tough penalties to reinforce the message.

Governors and council officials were alarmed at the prospect of jail sentences, while heads said the NASUWT had to realise that some schools did not have the cash to implement the deal. But Ms Keates does not believe prison is unduly harsh. “Not at all if the law is constantly being flouted.” The Department for Education and Skills said the deal would continue to be monitored by key stakeholders, including the NASUWT.