A headteachers’ union is to write to schools making clear they have a legal duty to teach about same-sex families in primary schools and about sex education in secondary schools.
The letter from the NAHT will tell schools they should do this “sensitively and involve parents in discussions to explain their lessons”.
The intervention follows parents protesting over the way schools have taught children about LGBT people in primary schools.
Quick read: Protests at school over LGBT lessons row
Quick read: Spielman backs school over No Outsiders
Background: Teacher shortlisted for global prize
Paul Whiteman, the general secretary of the NAHT said: "Teachers should be supported and protected for teaching tolerance. It’s not just what the law says, it’s the right thing to do.”
The union chief warned that in some cases children have been left scared to go to school.
Writing in The Sunday Times, he said: “Schools should be a place of safety and calm, and everyone in the community has a duty to create and preserve that atmosphere.
"Protests do nothing to help schools achieve their public duty or create the conditions children need to learn. The protests are irresponsible and they should stop."
In recent weeks, Parkfield Community School in Birmingham has been at the centre of controversy over its No Outsiders programme of lessons, which teaches pupils about diversity, acceptance and equality.
Some parents at the majority-Muslim school have held demonstrations against the lessons, and against assistant headteacher Andrew Moffat, who organised the project.
Protests have also been taking place outside Anderton Park Primary School, in the same area, with parents demonstrating outside, waving banners and calling for the lessons to stop.
Last month, the BBC Asian Network revealed that parents in Manchester, Oldham, Blackburn and Bradford were being organised to campaign against LGBT content.
Mr Whiteman said: “The fundamental British values of democracy, the rule of law, individual liberty and mutual respect, and tolerance of those with different faiths and beliefs are enshrined in law.”
He added that protected characteristics, according to the law, are race, disability, sex, age, religion or belief, sexual orientation, pregnancy and maternity, and gender reassignment.
“The Equality Act says that schools should eliminate discrimination by promoting tolerance and friendship and by sharing an understanding of a range of religions or cultures.
Each of the protected characteristics I have listed are of equal status. The law does not permit schools to pick which ones it educates pupils about. All must be included.”
The NAHT held emergency talks with education department officials last week, according to The Sunday Times.