The survey of 3,200 secondary school teachers by YoungMinds found that 84 per cent have taught a child suspected of self-harming.
However, around only a third feel confident in their ability to support young people with mental health issues such as self-harming, while a similar proportion admitted they lack confidence to help.
Only 39 per cent say they know how and when to refer young people to child and adolescent mental health services, with 37 per cent admitting they lack confidence to alert specialists.
A lack of training has emerged as a key barrier to teachers providing help - 77 per cent told the charity they have not had sufficient training on children's mental health.
YoungMinds said the survey's findings are particularly concerning as students with mental health problems often turn to teachers for support.
Last year, research from NHS Digital found that 48 per cent of young people facing such issues sought help from their teachers.
"The vast majority of secondary school teachers have first-hand experience of teaching a child who is self-harming, but too often they haven't had enough training or support to know how best to respond to their need," said YoungMinds chief executive Emma Thomas.
"The reasons why a young person may self-harm are multiple and complex, but it's vital that when they reach out to someone about it, they are met with the best possible help and advice.
"We know that teachers do a great job of supporting young people in many instances, but it can also be hard to know what to say, when to take action or how to support someone."
The survey findings have been released to coincide with Self-Harm Awarness Day (1 March). YoungMinds has created an online guide for teachers and school staff to help them better support young people they believe are self-harming.
In addition, young people's support charity The Mix has launched a video campaign to raise awareness of self-harm issues, featuring young people affected by the issue.