Four in five teachers have seen an increase in student mental health problems, with one saying nine-year-olds are talking about suicide.
A survey by the NEU teaching union suggests 83 per cent of teaching staff saw a rise in the past two years.
Seven per cent said they had not noticed a change, and 11 per cent could not be sure.
One of the 8,674 members surveyed said: "Sats pressure and general expectations are taking their toll on more vulnerable pupils.
"We have nine-year-olds talking about suicide."
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Another commented: "Much more anxiety, self-harming. Three suicides in three years in my school alone."
Some of those questioned said student mental health was at a "crisis point", with others saying it was affecting "younger and younger children".
The staff were also asked whether their workplace had the provision for supporting pupils with mental health issues.
The majority of teachers (59 per cent) said they had learning support assistants, 49 per cent said they had a school counsellor, and 30 per cent identified external specialist support.
Twenty-nine per cent reported having a school nurse, and 12 per cent said they had a mental health first aider.
One NEU member said: "I spend most lunchtimes and 40 per cent of my time nurturing children experiencing a range of mental health issues.
"I am currently working with 15 children who have been bereaved, have anxiety, have PTSD or a parent with a terminal/life-threatening illness."
Another said they had lost the school counsellor due to a lack of funds.
The teaching staff was also asked what stops them from supporting young people who are experiencing mental health issues.
In the multiple choice responses, 57 per cent cited real-terms funding cuts, 51 per cent said a reduction in teaching assistants and learning support assistants (40 per cent).
Other factors were the narrowing of the curriculum (32 per cent), the assessment system (53 per cent) and personal workload (64 per cent).
One teacher described the situation like a "slow-motion car crash" that they were powerless to stop.
Kevin Courtney, joint general secretary of the NEU, said: "These are alarming reports of a growing crisis in our schools and society.
"It is very clear that this government's policies on education and school funding are contributing to a terrible and destructive situation for young people and the education workforce.
"Schools can't solve this alone and government's underfunding of public services is damaging the next generation from an early age."
A DfE spokesperson said: “Mental health is just as important as physical health and should be treated as such.
“That is why the education secretary has made children's mental health a key priority for this government; and through our new compulsory health education all children will be taught how to look after their mental wellbeing and recognise when classmates are struggling.”
They added that by 2023-24 an extra 345,000 children and young people up to the age of 25 will benefit from a range of mental health services.