etta81, the thing is that children are not brought up in a vacuum with contact only with their teachers and their parents - as the poster said above, even if they are home educated they are not out of contact with other children. They are part of a peer group, and it is definitely worth making sure that they all have some basic information and some basic ground rules.
My parents didn't do any sex education at all. None, nada, zip. Not body parts, not relationships, nothing. My school showed us a picture of chickens mating and then went straight to Tampax (literally - the film was sponsored by them - it was a film about tampons rather than about periods), sexually transmitted diseases and contraception (in a very, very pre-AIDS format).
Simultaneously I have grown up in a peer group where there was plenty of porn around (even in the 70s), losing your virginity aged 17 was average and where as far as the media were concerned it was pretty abnormal not to have sex with your partner straight away, and where an awful lot of negative 50s/60s/70s propaganda about women being 'teases' was still around, and that being a tease was the worst thing you could be (consider Grease the film - Sandy is only OK once she puts out, basically).
I had absolutely no way of reconciling these views. My parents, I think, felt that sex was for marriage only, but they never told me that! My mum was shocked and disgusted when I told her aged 17 that I was going to the family planning clinic - I naively thought she would be pleased at my responsible attitude! They never asked what I was being taught at school, what relationships my friends were having, never discussed what view of relationships I was building up.
this is a ramble - my point is that your children WILL learn about sex and relationships from an early age whether you talk to them about this or not. It's actually very good that the school has a plan to teach things from an early age in a staged way, rather than plunging straight in to serious stuff as my school did. However, the biggest influence on your children, especially at this young age, remains you, and the more open you are, the better. I would say don't remove them from the lessons - discuss the lessons with them, and challenge anything you don't like. Go and talk to the teacher and get more information.