Under proposals set to be outlined in a green paper tomorrow (Monday), every school and college in England will be "incentivised" to appoint a designated senior lead for mental health to co-ordinate existing school-based support as well as helping children to access specialist therapies and other NHS treatments.
Supported by a training package of up to £95m from 2019, the senior leads will also be responsible for developing a "whole-school approach" to mental health and wellbeing.
This will include making sure pastoral support is available for all pupils and that strong policies are in place to reduce bullying and other behaviours that can cause mental distress.
A further £215m will be available to create new "mental health support teams" which will aim to improve joint working working between schools and the NHS.
The teams will provide a wider range of support and treatments in or near schools and colleges, to improve early intervention so mental health problems can be addressed before they become more serious.
The government said several thousand people are likely to be recruited over the next five years to form the new teams, which could be trained to offer cognitive behaviour therapy and other evidence-based treatments in classrooms.
Supervised by clinicians they will also work closely with educational psychologists, school nurses, counsellors, social workers and others to assess and refer children for other specialist treatments if necessary.
Other measures set out in the green paper include ensuring every primary and secondary school in the country is offered mental health awareness training and making mental health and wellbeing a focus of efforts to improve the quality of relationships education and PSHE.
Meanwhile, a new working group will be established to look at mental health support for 16- to 25-year-olds and further research will be commissioned to fill evidence gaps across children's mental health, including understanding how better to support vulnerable families.
Health Secretary Jeremy Hunt said: "Around half of all mental illness starts before the age of 14 so it is vital children get support as soon as they need it - in the classroom. If we catch mental ill health early we can treat it and stop it turning into something more serious.
"These ambitious new plans will work with schools to make sure this happens, as well as reducing waiting times for the most severe cases."
Education Secretary Justine Greening said there are already great examples of schools and colleges across the country playing a vital role in supporting students' wellbeing and mental health.
"We want that kind of excellence to become the norm and these proposals will help deliver that by strengthening the links between schools and the experts who can give young people the support they need," she said.
Sarah Brennan, chief executive of YoungMinds, said too many children are not getting the support that 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.
"We are very pleased to see the government recognise the fundamental importance that schools play in building resilience of their pupils and intervening early when problems do emerge. So we welcome the green paper's proposals to introduce mental health leads in every school, as well as mental health support teams to offer support within schools as early as possible.
"The ambition for a four-week waiting time is also welcome. Long waits have a devastating impact on young people and their families, and currently only one in four young people with mental health problems get the help they need. Now it is crucial that services are given the resource to match the true scale of need, so that all children and young people in need of mental health support are able to get it."
The government has already committed £1.4bn in funding to improve mental health provision for children and young people up to 2020, although there are concerns that clinical commissioning groups (CCGs) are siphoning off this money to backfill cuts or spend on other priorities.
Anna Feuchtwang, chief executive of the National Children's Bureau, said the proposals could mark an important milestone in how children access mental health support.
"Although much will depend on the detail, the emphasis on prevention and early intervention at school is particularly welcome," she said.
"The government should implement these reforms without delay, so that children don't have to wait a day longer than necessary for the help they need."
Feuchtwang said that new funding for a senior member of staff to lead on mental health work could help schools to focus on pupils' psychological welfare and happiness, and tackle other pressing issues such as bullying.
She added that four-week waiting time targets will help restore young people's faith in the system, and send a message that mental health is as important as physical health.
"However, as NHS England recently made clear, the health system is struggling to meet existing targets, emphasising the need for this new commitment to be backed up with adequate resources," she said.
Izzi Seccombe, chair of the Local Government Association's community wellbeing board, said measures to tackle the "crisis" in children and young people's mental health are long overdue.
"The reality is that all children need support and we hope this can be extended to all schools," she said.
"This support needs to be available on the same day, at the point of request. Councils have previously called for an independent counselling service in all schools, as evidence shows that children and young people find it a crucial support, and such a service would reduce overall demand on specialist services, too.
"Today's measures are a step in the right direction, but we need a root-and-branch overhaul of children's mental health services if we're to develop a system that says yes, rather than no, to children when they ask for help."
Jaime Smith, programme director for mental health and wellbeing in schools at the Anna Freud National Centre for Children and Families, said: "The publication of this green paper is an important step in acknowledging the role that schools play in promoting the mental health and wellbeing of children and young people.
"It is vital now that the ambition of the green paper is carried forward and that schools, colleges and early years providers are equipped through support and funding to feel confident in promoting positive mental health and wellbeing and ensuring that those children and young people who are at risk receive timely and appropriate help."
A consultation on the green paper will run for approximately 13 weeks.