One in five gay pupils said they had received death threats from classmates.
Gay pupils at faith schools suffered particularly badly, with 75 per cent being homophobically bullied. They were also far less likely to tell teachers they were gay.
Half of all teachers failed to act on homophobic language, according to the survey of 1,145 gay teenagers published by Stonewall, the gay rights organisation.
Nearly two-thirds of pupils said there were no adults at school they could talk to about being gay. One boy was told by his teacher that anyone who came out as gay "would have to be reported". Several were told that the bullying was their own fault for telling people that they were gay.
Half of those bullied had played truant at least once. Holly Anderson, 19, regularly missed lessons at her Hampshire secondary, after coming out at 14.
"I got things chucked at me in corridors, there was name-calling, people spat at me," she said. "Teachers said it was just teasing."
Once she was punched in the face as she came out of the lavatory, and was then told off for fighting.
The NUT said: "Nobody should stand by and allow another person to suffer.
Putting a stop to such behaviour just takes determination."