Many teachers seriously misunderstand eating disorders and self-harm, allowing the problems to develop unchecked.
New research, carried out at the Institute of Psychiatry in London, reveals that even when teachers are able to spot potential problems, they lack the training to follow through effectively.
Pooky Hesmondhalgh, who is conducting the research, cites the example of a pupil admitted to hospital after hitting a major artery during a self- harming episode.
"Six or seven members of staff said they thought something was up," she said. "But they didn't do anything."
When untrained staff do take action, it can be equally problematic. One Year 9 pupil was observed to be eating very little at lunchtimes.
"She was told, `We're watching you,'" Ms Hesmondhalgh said. "So she stopped going to lunch altogether. Children aren't stupid. They find ways around you. It's about being aware and approaching things in the right way."
In an average, co-educational secondary with 1,000 pupils, 27 are likely to have an eating disorder; about 80 will harm themselves.
But teachers often have little understanding of the motivation behind the two behaviour patterns and are unaware that both can be methods of coping with overwhelming emotions.
Self-harm, in particular, tends to fill people with horror. One teacher was so repulsed by a pupil who confessed to self-harming behaviour that she severed all contact immediately.
"The best person to help a child move on can be a member of staff they've got to know," said Ms Hesmondhalgh. "But teachers are scared they'll do something wrong and make things worse.
"You can't expect staff to know about everything. They're responsible for so many aspects of children's lives. But, in this case, they're really in a good position to help."
She believes teachers should be trained to identify problems early, and to respond appropriately. They also need to know how to work with parents, who can be reluctant to admit that there is a problem.
Hannah Smith, of the mental health charity Young Minds, agrees. She would like to see a compulsory mental health module included in teacher-training courses and regular professional development sessions.
"People see these conditions on soaps and the news," she said. "But they don't necessarily know what to do or what advice to offer.
"Young people say the first person they would go to would be their teacher or youth worker. And worried parents go to teachers as well. So it seems like we're missing the first port of call there."