Dr Heledd Hayes, the union's education officer, said: "We want new teachers to prepare for the worst. We are not trying to scare them but unfortunately it is an aspect of life in schools."
More than 1,200 cases have been dealt with by the union in Wales in the past eight years. Only 4 per cent resulted in a conviction. No further action was taken in three-quarters of cases.
The ordeal facd by teachers accused by pupils has been highlighted by the case of Monmouthshire headteacher Marjorie Evans, who fought a 13-month battle to clear her name after being accused of slapping a 10-year-old.
In the wake of the case, the NUT has threatened industrial action, including strike action, if heads fail to exclude permanently any pupils whose behaviour requires them to be physically restrained.
New guidance on how to deal with unacceptable pupil behaviour has been distributed to the union's 202,000 members in England and Wales.