Too many pupils with symptoms of depression or anxiety are let down or ignored, claims charity MindFull.
It has launched a new online counselling service to support and advise 11- to 17-year-olds.
MindFull's founder Emma-Jane Cross said poor mental health among young people was "one of the last great medical taboos in the UK today".
The charity's inaugural report calls for a "sea-change" in approaches to young people's mental health.
In her foreword, Ms Cross describes the scale of the problem as "epic" with thousands "teetering on the brink" of serious mental illness, risking "terrible long-term effects" for both individuals and society at large.
Too many "are having to resort to harming themselves on purpose in order to cope, or worse still are thinking about ending their own lives", she added later.
The report calls for awareness of the importance of mental health to be integrated into every aspect of young people's development particularly in schools and health services.
"We need to move away from only tackling the symptoms of acute poor mental health and well-being to focus on education, prevention and early intervention," say the authors.
"Young people need to be encouraged to speak out about their mental health and well-being and feel confident that, when they do, they will receive the support they need as swiftly and as easily as possible."
The charity says its new online support service will help do this by providing professional counselling and peer support for young people.
It says it will also be working with schools to educate young people on how to cope with mental health issues.
The charity has the backing of Labour leader Ed Miliband and of campaigners for better mental health support for young people.
Psychologist Prof Tanya Byron said: "Just as we look after our children's physical health, it's vital we offer support for their mental well-being.
"Children and young people are clearly not getting the help they need. That's why this new online support from MindFull is so important."
Lucie Russell of the YoungMinds charity said: "Children and young people are growing up in a toxic climate. They exist in a 24/7 online world where they never switch off, where cyberbullying, consumerism and pornography, sexting and the pressure to have the perfect body bombard them daily, where any exam grade below a C means failure and employment prospects are bleak.
"We know from our extensive work with young people that the support they so desperately need when they aren't coping is grossly lacking but we also know from services that they are under huge funding pressures and are overwhelmed with demand."
Health minister Norman Lamb said the government was "placing an unprecedented emphasis on mental health", particularly for children and young people.
"More than a third of children and young people now live in an area where children and adolescent mental health services have been transformed.
"We are investing £54m into improving access to mental health treatments for children and young people."
Mental health lessons should be on the timetable in every secondary school in the UK a new charity has urged.