Teachers who have been given special training to deal with pupils' mental health problems will make a "real difference to children's lives", Theresa May said.
Staff have begun to receive the training as part of the Prime Minister's drive to tackle the "burning injustices" in society.
Over the next three years some 3,000 staff, covering every secondary school in England, will receive advice on how to deal with issues such as depression and anxiety, suicide and psychosis, self-harm and eating disorders.
The programme, announced in January, is delivered by social enterprise Mental Health First Aid (MHFA) England, backed by £200,000 in Government funding and will be extended to primary schools by 2022.
Mrs May said: "When I stood on the steps of Downing Street on my first day as Prime Minister, I said that the disparity in mental health services was one of the burning injustices our country faces.
"Since then we have announced real progress in tackling this unfairness, and this training will make a real difference to children's lives by ensuring they have access to sensitive and swift support.
"Tackling poor mental health is a huge challenge, and we will keep our promises and meet that challenge with the comprehensive cross-society response that is required."
Sarah Brennan, chief executive of mental health charity YoungMinds, said: "Children and young people today are facing a huge range of pressures, from exam stress to online bullying, which inevitably take a toll on their mental health.
"Many of these pressures become particularly intense during secondary school so it is important and welcome that mental health first aid training will be available for secondary schools.
"This training is a move in the right direction and will help give staff the opportunity to gain confidence and understand mental health better.
"We hope it will encourage more leadership teams to put student wellbeing at the heart of their school which will benefit both students and schools alike.
"Young people need to learn about wellbeing and resilience from a young age, so when they leave school they are equipped to deal with problems and have the confidence to seek help.
"It is vital that this work is part of a whole-school approach to wellbeing, and that mental health is made a priority across the education system."
:: The first schools to host the training are: Broadland High School, Hoveton in Norfolk; Patcham High School, Brighton; King's Leadership Academy, Liverpool; Blessed William Howard Catholic School, Stafford; Ormiston Endeavour Academy, Ipswich; Shirebrook Academy, Shirebrook in Derbyshire; Bristol Metropolitan Academy, Bristol; Oasis Academy - Shirley Park, Croydon, south London; Orchard School, Bristol; Haberdasher's Aske's College, Lewisham, south London; Trentham High School, Staffordshire.