Referrals to child mental health units from UK primary schools for pupils aged 11 and under have risen by nearly 50% in three years, the BBC has learned.
Replies to Freedom of Information (FOI) requests from 46 health trusts indicate they rose from 21,125 to 31,531.
Seven trusts said they had rejected an individual pupil for treatment at least five times over the last four years.
The government says it is "determined to improve mental health support".
Pupils had also spent more than a year on a waiting list for mental health services at 12 different trusts.
"These figures are deeply worrying and build on evidence which shows emotional disorders in children have increased in recent years," said Dr Bernadka Dubicka, chairwoman of the child and adolescent faculty at the Royal College of Psychiatrists.
"Services for children have been historically underfunded meaning they are unable to meet increased demand," she added.
"The government's aim to provide mental health support in all schools within the next 10 years will be too little, too late for many children who need that help now."
Head teachers have told the BBC that the number of serious mental health episodes is on the rise in their schools.
One pupil was rejected for treatment on nine occasions and another spent nearly three years on a waiting list.
"We're seeing an increase year on year - more and more children with a variety of problems and it just seems to be getting harder and harder to manage", said Dani Worthington, head teacher of Moorside Community Primary School in Halifax.
"We've seen children headbutting walls, punching walls, kicking walls, and this can sometimes happen on a daily basis for these children as they're going through some sort of crisis."
Sue Blair, head teacher at Pennine Way Primary School in Carlisle, said: "I think the crisis is really acute."
She added that she is seeing self-harm in seven and eight year-olds, and pupils struggling with online bullying and eating disorders before they reach secondary school.
BBC News also sent FoI requests to 500 primary schools in England about serious mental health episodes.
It found that 191 primary school pupils had self-harmed on school grounds in the last four years, according to responses received from 155 schools.
These responses, and an account from a further school, also revealed that four pupils have attempted to kill themselves on primary school grounds over the last four years.
What to do if you are worried about your child
Although children often feel low from time to time, if your child is feeling unhappy and low for a prolonged period of time, it is time to seek more professional help.
Any professional working with children and young people should know what to do - for instance a teacher, school counsellor or welfare worker.
If the problem is complex, they may suggest approaching a specialist.
GPs can refer young people to specialist child and mental health services, or parents to a parenting programme.
Head teachers say securing mental health support for pupils can be a real challenge.
Clem Coady, head teacher at Stoneraise School in Carlisle, says he knows of a pupil "experiencing extreme mental health distress" who has been waiting two years for an assessment.
"I find it really abhorrent, there's nothing that we can realistically do that is going to give the child the help that child needs."
A government spokesperson said: "We are determined to improve mental health support for children and we are transforming services through the NHS Long Term Plan - backed by an extra £2.3bn a year - so that 345,000 more children and young people have access to specialist mental health care by 2023-24."
It said its mental health support teams are "training a new dedicated mental health workforce for schools and colleges across the country".
If you need support with mental health, help and support is available from the BBC Action Line.