Pressure from exams and schoolwork is the main reason for feelings of depression and anxiety in children.
A new report by the Ombudsman for Children found 78% said pressure in school causes them to have low moods. Body image (65%) was the second most common response for feelings of depression or anxiety.
2,166 children aged between 12 and 17 were surveyed and asked about their current mental health, the causes of mental health problems, where they got information on mental health, and their views on mental health services.
Six in ten children who had attended Child and Adolescent Mental Health Services (CAHMS) in the past year said they had difficulty accessing the service.
The report also found nearly half of those say the service they finally received didn't help with their mental health issues.
Children's Ombudsman Dr Niall Muldoon, says it's completely unacceptable that children have to wait long periods of time to access services.
"Especially on the back of some of the other recent reports from the Mental Health Commission as well, that CAHMs isn't working the way it should do.
"[This report] highlights the range of difficulties that children with mental health have, and that even when they do wait a long time, the service they get isn't suitable for their needs, or isn't satisfying them".
The 'Piece of My Mind' report found only 22% said that their mental health was good with half saying that they "sometimes felt low, stressed or anxious but that they could manage it themselves". Over one-quarter (28%) said their mental health was "not good and that they needed help to manage it".
More girls than boys said that they sometimes felt low, stressed or anxious (55% of girls compared to 44% of boys) or that their mental health was not good (60% compared to 31%).
The report also found mental health appears to get worse with age, with 16-17-year-olds most often reporting that their mental health was "just okay" or "not good".