Of 420 ATL members who responded to a poll, almost half said pupils in their school had self-harmed and 89 percent said testing was the main source of stress.
Some staff said they were aware of pupils attempting suicide |― and 18 of these were in primary schools.
The government says children's mental health is a priority.
Teaching staff complained the pressure placed on children of 10 and 11 by end of primary school tests was excessive.
Others blamed the government for raising the stakes on testing and exams, affecting some of the youngest children in their schools.
"Mental health issues are probably our biggest barrier to academic progress," said the head of a Norfolk primary school, ahead of the second day of debate at the ATL's annual conference in Liverpool.
"As head of school I am spending more and more of my time supporting children with mental health issues," added the head, who added that the member of staff responsible for pastoral care was 'now snowed under'.
A member of staff at a secondary academy in London reported a huge increase in physical symptoms of stress and incidents of self harm.
While suicidal thoughts have escalated beyond control.
The teachers who took part in the survey also pinpointed fragmented home-lives, family break-up and social media as major factors contributing to pupils' mental distress.
Almost three quarters (73 percent) felt young people were under more pressure now than two years ago ― while 82 percent felt students were more pressured now than 10 years ago.
A fifth (21 percent) of those surveyed said their school had no access to a counselor for pupils, but many others said their schools' counselors were overstretched with education staff having to shoulder the responsibility despite not being qualified to do so.
"Teachers are not social workers, psychologists or therapists but are increasingly expected to fulfill these roles," said a primary teacher from Somerset.
ATL general secretary Mary Bousted described the findings as horrifying.
"The government bears responsibility for much of this stress which appears to stem from a test-focused, over-crowded curriculum," said Bousted.
The overwhelming majority (93 percent) of the staff who took part were in England, with smaller numbers in other parts of the UK.
A Department for Education spokeswoman urged all schools to ensure pupils were supported at times of pressure.
"Tests are a key part of ensuring young people master the skills they need to reach their potential and succeed in life.
"But we have taken real steps to ensure they are not on a constant treadmill of revision and testing, including scrapping January modules, decoupling AS-levels and removing re-sits from league tables."