A seven-year-old boy threatened to stab a pregnant teacher in the stomach over a literacy lesson, it emerges as a union warns of a rise in children who do not respond to any form of punishment.

Pupil indiscipline and violence is a “significant and worsening problem” in schools, according to the National Association of Schoolmasters Union of Women Teachers (NASUWT).

Addressing the delegates at the union’s annual conference in Belfast, David Baxter, a member of the union’s executive in Northern Ireland, said teachers need to find a way to deal with “Teflon kids – the ones that no sanction seems to stick to”.

John Devlin, another member from Northern Ireland, described some of the shocking examples of aggressive and violent threats teachers have been subjected to. One pregnant teacher was told by a seven-year-old boy who did not want to do literacy “I’m going to stab you in your pregnant belly”, he said.

Meanwhile, another teacher was left with a bleeding neck from a pupil attack and a third was punched in the face by a ten-year-old boy.

Russ Walters, the union’s honorary treasurer, said: “We cannot continue to put up with this lack of dignity, this absolute prostitution of our professionalism.”