The survey of more than 300 secondary teachers in England, Scotland and Wales found 66 per of women and 41 per cent of men felt unsafe in their classrooms.
More than two-thirds said they would have to rely on a child to get help in an emergency.
"I don't feel safe," said a woman teacher at a London school. "I teach in a hut, so if I was in trouble or needed help I really don't know what I would do," Teachers were also concerned about their ability to react to emergencies.
Almost two-thirds had dealt with a medical emergency in the classroom. One teacher had to deal with a child who began to choke in a lesson.
"I had no way of contacting help, so I sent another child to find someone while I tried to deal with it on my own," he said. "I should have been able to raise the alarm much quicker."
Others reported that it might take 10 minutes to get help. In one school a teacher who taught in a classroom separated from the main building said she would have to ask a child to walk around to the front entrance of the school.
"It could take them up to 10 minutes because they don't have keys - they would have to go the long way round," she said.
The survey, carried out for the security company Radun Controls, follows widespread concern about teachers' safety after a London secondary teacher was raped in her classroom by a pupil.
Ministers have set up a leadership group on behaviour and discipline to look into the issue, with the protection of teachers as part of its remit.
Others who work in isolation routinely carry personal alarms, and a local authority health and safety officer said that schools should consider issuing them to staff, especially if their classrooms are isolated or if they are likely to be on the premises at the end of the school day.
"This research highlights the need for teacher reassurance," said Roger Watson, general manager of Radun Controls, which makes electronic registration equipment based on a swipe-card system.