Peter Smith, in his address to the union's assembly, said the profession was "like a hedgehog in denial - instinctively prickly".
"It is a challenge for teachers to talk teaching up. Whatever they are learning, children are clearly taking in the message that teaching can seriously damage your health, your sanity or your family life," he added.
A MORI poll of pupils published by the union found that 7 per cent of secondary pupils said they were very likely to consider teaching as a career, but only 4 per cent of boys said so.
The most common reason for the lack of interest in teaching, given by 62 per cent of the 4,000 pupils surveyed, was that they considered it too stressful.