Children suffering from anxiety and depression will be offered counselling at school under government plans to tackle a widely reported crisis in young people’s mental health. Pupils in England will be able to attend sessions with therapists at school or college in an attempt to stop any psychological difficulties deepening into lifelong issues.

Every school will also be required to appoint a teacher to co-ordinate improved support for the fast-growing number of children who are struggling mentally, many self-harming as a result of bullying, exam stress, dissatisfaction with their body shape, troubles at home and other factors.

The plans are included in a government green paper to be launched on Sunday by health secretary Jeremy Hunt and education secretary Justine Greening.

A new guaranteed maximum four-week waiting time for children with more complex problems to access NHS child and adolescent mental health services (Camhs) will be phased in. That is a response to concerns that many vulnerable under-18s, including some who may be suicidal, are being forced to wait for care or even denied help because Camhs care is overloaded.

“Around half of all mental illness starts before the age of 14 so it is vital that children get support as soon as they need it – in the classroom. If we catch mental illness early we can treat it and stop it turning into something more serious,” said Hunt.

Sarah Brennan, chief executive of the charity YoungMinds, welcomed the plans. “We are facing a crisis in our classrooms and far too many children are not getting the support they need. Too often we hear from young people who have started to self-harm, become suicidal or dropped out of school while waiting for the right help,” she said.

The improvements will begin in 2019 and be backed by what the government says is £300m of new funding over several years, which is on top of the £1.35bn the coalition government allocated to children’s mental health up to 2020.

The National Association of Head Teachers, which represents most primary school heads, welcomed the four-week Camhs waiting time as an “extremely important step forward”. Under-18s are currently enduring waits of as long as 18 months, the NHS regulator said recently.

Around £215m of the £300m will fund the creation of mental health support teams in schools. Ministers intend that several thousand new “children and young people’s wellness practitioners”, therapists providing mainly cognitive behaviour therapy, will undertake most of the work with pupils, but with school nurses and educational psychologists also involved. Ministers hope that this increase in early intervention will reduce the number of children who go on to struggle mentally as adults.

However, the initiatives will initially be piloted to assess their effectiveness, so the new forms of support envisaged will not be available across England until an unspecified time in the 2020s. The government’s ambition is only that they have been put in place in a fifth of the country by 2022-23.

Dr Bernadka Dubicka, a children’s psychiatrist who chairs the Royal College of Psychiatrists’ child and adolescent faculty, said she was frustrated that more help would not be put in place sooner. While welcoming the four-week treatment pledge, she also queried where the extra mental health professionals would come from to provide speedier Camhs care. Official figures show that the number of specialist children’s psychiatrists working in the NHS in England has fallen since 2013.

Conservative MP Sarah Wollaston, chair of the health select committee, welcomed the announcement but said she was keen to see more details. “We need to have a much greater focus on early intervention and prevention. Any money going into that is a good thing,” she said.

She welcomed better coordination between schools and the NHS, but said that some schools were already working well with the health service and others should learn the lessons from those places. “It’s often down to resourcing,” she said. “Most young people prefer to have these services delivered in a setting of school because it’s much easier to access. Children don’t necessarily want to feel stigmatised by a referral to psychiatric services.”

Catherine Roche, chief executive of the national children’s mental health charity Place2Be, said: “We welcome the commitment demonstrated by the green paper, and are heartened to see recognition of how vital it is to provide mental health support in schools. We believe that a ‘whole school approach’ to mental health is essential to build a culture of openness and understanding, with appropriately qualified mental health professionals available when needed.”

Liberal Democrat MP Norman Lamb, the coalition’s mental health minister, said: “We published the Future in Mind report in March 2015. It was a blueprint for modernising children and young people’s mental health services. They have failed to drive the implementation of that blueprint. Why should we have any expectation that this will be any different? They should have just implemented it. At its heart it was all about linking schools much more closely with mental health services. Two years on, deja vu.”

Barbara Keeley, Labour’s shadow cabinet minister for mental health, said that the plans left “many unanswered questions”, including over funding and whether every school would be able to help every pupil who needed it. “The Tories’ record on children and young people’s mental health has been shocking, with a postcode lottery of provision across child and adolescent mental health services and many long waits for treatment,” she said.