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

Teachers’ union the NASUWT will call on trade unionists at next month’s Trades Union Congress in Brighton to back jail sentences and substantial fines for schools which deny teachers their rights to better working conditions.

The Westminster government has already told teachers they can take their employers to tribunals if they flout the workforce deal, which was agreed two years ago.

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

Latest figures show that all but 10 primary schools in England have now implemented the deal at least in part.

But Ms Keates believes a significant number of schools in Wales have yet to fully put into practice the first phase of the deal, which removed administrative tasks such as photocopying and putting up displays from teachers.