The government is being urged to issue new guidance on Relationships and Sex Education (RSE) over concerns about the content of the curriculum.
The concerns were raised in a letter from The Family Education Trust to schools minister, Robin Walker.
"Children are being sexualised, labelled with LGBT concepts that they are too young to comprehend, put at risk of abuse – and are being indoctrinated with gender ideology," the Trust said.
It said "so many" parents had been in touch after finding that senior leaders at their children's schools were ignoring their concerns about "inappropriate" RSE and refusing to discuss the curriculum.
The Trust also voiced concern about schools inviting "unsuitable" guest speakers, including trans activists, and said it was aware of "countless occasions" where RSE materials promoting gender ideology or contradicting the 2010 Equality Act were being used in schools.
In his reply, Mr Walker suggested that the curriculum was an important part of schools fulfilling their role in "preparing pupils for life in modern Britain".
"This involves supporting pupils to understand the society in which they grow up and teaching about respect for other people and for difference," he said.
Mr Walker said there was a need to teach pupils "from an early age about the concept of consent in an age-appropriate way", and "to help younger children understand what healthy relationships look like and the importance of respecting others' boundaries and privacy".
"This includes ensuring young people understand that they have rights over their own bodies and equipping them with the knowledge they need to recognise and to report abuse, including emotional, physical and sexual abuse," he wrote.
He went on to say that schools should check visitors' credentials but added that the Department for Education "cannot advise schools on which resources, external speakers or organisations will be the most suitable for schools to use, not least because schools operate in a variety of different contexts and have both the expertise and knowledge that makes them best placed to make these decisions".
The Family Education Trust said it was "disappointed" by Mr Walker's response and that it would continue to campaign for new guidance for schools "in order to protect children from sexualisation".
"Children should be allowed to enjoy their childhoods, not be introduced to adult sexual concepts and pseudoscience about gender identity within schools in the UK. Children and their parents deserve better," the Trust said.