Schoolchildren as young as four will be taught about relationships, keeping safe on the internet and looking after their own mental health as part of dedicated lessons in classrooms from next year, under new guidelines announced by the Government.

Pupils will learn the link between physical and mental health, with lessons focused on the importance of getting enough sleep, the dangers of sexting, and spotting anxiety in their friends.

Three new subjects – relationships education from primary school, relationships and sex education at secondary school, and health education for all ages – will form part of the school curriculum in England from 2020, with teachers deciding how frequently to hold age-appropriate lessons.

It will coincide with the first shake-up of sex education for two decades.

Education Secretary Damian Hinds said: “Growing up and adolescence are hard enough, but the internet and social media add new pressures that just weren’t there even one generation ago.

“So many things about the way people interact have changed, and this new world, seamless between online and offline, can be difficult to navigate.

“Almost 20 years on from the last time guidance on sex education was updated, there is a lot to catch up on.”

Mr Hinds said it was “appropriate” to make health education universal alongside relationships and sex education.

“It will help children learn how to look after themselves, physically and mentally, and the importance of getting away from the screen and the headphones,” he said.

“And it can help young people be resilient as they chart a course through an ever more complex world.”

Under the plans, children as young as four will be taught self-care – such as getting enough sleep and spending time outdoors.

They will also receive lessons in online safety, such as what to do when they come across something they find uncomfortable, trolling, and chatting to strangers.

The announcement follows intense pressure for action in safeguarding vulnerable people, highlighted by the case of 14-year-old Molly Russell who killed herself in 2017. Her family found material relating to depression and suicide when they looked at her Instagram account after her death.

According to the curriculum guidelines produced on Monday, secondary school pupils will be taught about female genital mutilation (FGM) – focusing on awareness, the availability of support networks, and reminding them that it is illegal.

Students aged 11 and older should also be taught about other forms of honour-based abuse, as well as grooming, forced marriage and domestic abuse as part of a strengthened curriculum, the Department for Education said.

Teachers at secondary school will have to take lessons on online safety topics, including the serious risks of sharing private photos, the impact of viewing explicit or harmful content – including how to report it and get support – as well as how the internet can promote an unhealthy view of sex and relationships.

Anna Feuchtwang, chief executive of campaign group the National Children’s Bureau, described the guidance as “a welcome step forward in preparing children for adulthood, improving their well-being and keeping them safe and healthy”.

She said: “By providing compulsory health education with a strong focus on mental well-being, and guaranteeing relationships education in primary schools and relationships and sex education in secondary schools, the Government has responded to the needs and concerns of children, young people and parents.

“This is an important milestone but there is further work ahead to ensure the new requirements fulfil their potential for helping children grow up healthier and happier.”

Leethan Bartholomew, from charity Barnardo’s, said: “This announcement is therefore welcomed by the National FGM Centre, as it will give a certain group of children the opportunity to be made aware of this hidden form of child abuse.

“However, it must be acknowledged that most girls are cut at an age when they will be attending primary school. Therefore conversations about FGM should take place at a younger age.

“Whilst some may have reservations about this, the work of the FGM Centre has shown this can be done in a child-centred way that can achieve the intended outcome of safeguarding children.”

MPs will on Monday debate a petition calling for parents to have the right to opt their child out of relationship and sex education.

The petition, which has gathered more than 106,000 signatures, said: “We have grave concerns about the physical, psychological and spiritual implications of teaching children about certain sexual and relational concepts proposed in RSE and believe that they have no place within a mandatory school curriculum.”