School staff are increasingly refusing to teach unruly children, figures show, amid incidents of violence and sexual assault.

The number of “refusal to teach” ballots has doubled in the past five years, according to the teaching union National Association of Schoolmasters Union of Women Teachers (NASUWT).

Teachers at a comprehensive school in Cardiff refused to teach a Year 10 boy with a history of “disruptive, aggressive and violent behaviour” who sexually assaulted a female member of staff, the union said.

Meanwhile staff a secondary school in Tyne and Wear refused to teach a Year 9 girl who was becoming “more and more violent”, and had been urinating throughout the school, often in front of staff, as well as “defecating around the school [and] wiping it on school equipment”.

At an academy in Essex, teachers balloted for industrial action after staff said they no longer felt safe due to “physical and verbal abuse” from students and reports of a number of blades and weapons brought into school.

Under a “refusal to teach” ballot, staff in a particular school can take industrial action as a last resort, to protest against the behaviour of a particular pupil. They can also take action short of a strike, which could involve refusing to cover lessons for colleagues or take on voluntary duties.