David Hart, general secretary of the NAHT said: "It was very threatening for teachers. Sometimes the irate parents would knock at the school door to remonstate with the head before she had a clue about the incident."
Mr Hart said any pupil using a mobile in class to complain to their parents should have it confiscated. Some 3.6 million under-16s own a mobile. Until now, schools' main concern had been over the thefts and disruption they can cause.