Children as young as four are to become the youngest in Scotland to be given sex education lessons under a new curriculum from Glasgow City Council.
Lessons begin in P1 with children being taught about body parts and conclude in S6 with classes on sexually transmitted infection, especially HIV and Aids.
However, the Scottish Conservative Party said there would be "massive concerns" about the scheme.
It said parents would question the need to involve primary one pupils.
The new sexual health and relationships curriculum was piloted in 13 primary and two secondary schools.
The council said teachers, pupils and parents gave it "overwhelming support".
Maureen McKenna, service director of education, said the programme was devised to build knowledge year-on-year.
"What we have is a broad curriculum which deals with emotions, friendships, self-esteem and respect as well as physical development and sexual health," she said.
"From P1 onwards, topics and issues are introduced at an appropriate age and handled in an appropriate way. It is tailored to meet the needs and development of young people in Glasgow.
"Parents have a crucial part to play too and by keeping them involved through supporting their child's learning they are prepared for any questions that may crop up at home."
Following the pilot period in 2007/08, the curriculum was extended across other areas in the east end of Glasgow.
The roll-out is continuing in the north and west of the city during this academic year with plans now being put in place to extend the curriculum across the remaining areas of Glasgow.
Councillor James Coleman, who chairs Glasgow's young person's sexual health steering group, said the curriculum could be "a blueprint for others to follow".
"Both parents and children made it very clear to us they wanted to be closer to one another on this issue and this curriculum allows for that to happen," he said.
"Stronger bonds between parents and their child help to encourage young people to delay engaging in sexual activity until they are physically and emotionally equipped to deal with the consequences.
"What we are now rolling-out in Glasgow represents a significant step forward in the provision of sexual health and relationships education."
The Scottish Conservative spokeswoman for schools, Liz Smith, said there would be "massive concerns" about the new curriculum.
"Children as young as four - and just out of nursery - will be involved in this new curriculum," she said.
"Just how well can they be expected to cope with understanding even some of the more simple facts in what is a very complex and sensitive issue?
"Many parents will question whether this is really the most appropriate priority in primary one classes - especially when there is yet more evidence published today that Scotland is performing poorly when it comes to teaching children to read, write and count."