Almost half of all schoolchildren cannot name a single mental health condition, a study of 500 young people has found.
And fewer than half of those surveyed felt that schools provided enough information about mental health, a third preferred to trust the internet.
Great Ormond Street Hospital conducted the poll. It is launching its own child mental health information website.
A spokesman said the results were "worrying and surprising".
Mental health problems are far from rare in the young - between one in 12 and one in 15 children and teenagers deliberately self-harm, with more than 25,000 admitted to hospital each year due to the severity of their injuries.
More than 1% of 11 to 16-year-olds are said to be "seriously depressed."
However, the survey of 500 young people revealed wide gaps in knowledge, with boys between the ages of 12 and 14 the least likely to be able to name a single mental health condition.
Those who did manage to do so were most likely to point to substance abuse, depression and self-harm as the most common.
While the internet was the most popular source of advice, 17 and 18-year-old girls said that they likely to gather their mental health knowledge from celebrities talking about their mental health problems.
Dr John Goldin, a consultant child and adolescent psychiatrist at Great Ormond Street, said: "Our findings which show that nearly half of 12-18 year olds in the UK cannot name a single mental health condition are both worrying and surprising.
"This ignorance is probably one of the reasons why for too long now there has been a stigma attached to living with a mental health condition.
"It is important that young people feel they can come forward and speak out if they or someone they know is experiencing mental health difficulties."
An analysis of 2,000 confidential enquiries to Great Ormond Street's "Children First" website found that "psychosocial" problems were the most common topic.
The hospital has responded by setting up a new section on the site, complete with information and audio diaries recorded by teenagers with mental health problems.
Child mental health charity Young Minds said the website would be a "vital resource".
Its chief executive Sarah Brennan said: "Websites such as Great Ormond Street Hospital's help young people to understand more about mental health problems and ensure they have access to information when they need it.
"They provide information which young people don't feel comfortable getting elsewhere, often helping to identify what might be wrong and how to get help."
ignorant a. 無知的,不學無術的;沒有受教育的
deliberately ad. 故意地,蓄意地
stigma n. 恥辱,污名