The children's charity surveyed 5,555 secondary school pupils across the UK aged between 13 and 18, and found that 33 per cent have mental health and emotional problems such as depression, or a lack of sleep, motivation and focus.
Those surveyed took part in a research exercise for the charity's "blues programme", which offers mental health support to pupils.
The survey used the CES-D scale questionnaire, which produces scores from zero to a maximum of 60 to measure depression. A third of those who responded had a score of at least 20, which makes them eligible for help through the programme.
"It's troubling that so many of our young people are dealing with issues of depression and anxiety - and all too often alone," said Action for Children chief executive Julie Bentley.
"Every day at Action for Children, we support teenagers who are really struggling for a number of reasons. Young people are increasingly concerned about their futures as they become more aware of the political environment around them.
"Many are dealing with the intense pressure of school work, others are experiencing problems at home. Add in navigating an increasingly complex 24/7 world with constant stimulation from social media, and things often become too much for young people to handle.
"Getting help early can help stop some mental health problems in their tracks, but without quicker investment and targeted support from the government, many young people will continue to struggle.
"If they don't get the help they need early, we will see even higher levels of mental health problems as they reach adulthood."
The blues programme is run by the charity in 37 schools and over a six-week period uses cognitive behaviour therapy methods to help young people better understand negative feelings they have.
Between October 2017 and July 2018, 620 pupils took part in the programme. Of the 420 who answered an end-of-programme questionnaire, 70 per cent showed an improvement in their mental health and emotional wellbeing.
This includes 87 per cent showing an increase in self-confidence, 83 per cent saw a boost to their self-esteem and 82 per cent had better relationships at school, while 80 per cent felt more involved in lessons.
Among those who has been helped is 15-year-old Rowan, who attends Whitchurch High School in Cardiff, and suffered from low moods and anxiety.
"I couldn't really focus on things properly. There were scary periods where I was getting very anxious and not doing as well as I usually do at school as my mind was elsewhere. I tried to keep how I was feeling to myself and deal with the problems alone, but I didn't know what to do. My friends noticed a difference in me and kept asking me what was wrong," she said.
"It probably seems quite a small thing, but learning how to deal with my problems in a new way has made more of a difference than I could have imagined. I would be in a terribly dark place now if I hadn't learnt how to do it."
Action for Children says there has been a 52 per cent increase over the past three years in the number of children and young people presented to the charity with mental health issues.
In December 2017, the government's green paper Transforming Children and Young People's Mental Health promised £300m in extra funding to support children's mental health, with a particular focus on support in schools.
Earlier this month, Prime Minister Theresa May said that recruitment has begun for mental health support teams to provide support in primary and secondary schools.
May also announced that an annual review of children's mental health in England will take place from October 2019. She said she hopes schools will use this to help manage children's mental health.