My daughter is home schooled but I still think it is the responsibility of the parent. Find some good age appropriate literature and have them read it. Then sit down with them and discuss it using the literature as a guide. It is easier if they have a chance to read about it before you talk. It also is good to have some diagrams.
There is plenty of literature on the internet about it and this will give you a chance to straighten out some wild things that they get from their friends. My daughter had lots of that.
Be honest with them when they ask questions, and watch out when they are smaller and ask where babies come from. Don't give the whole sex ed thing when they ask where babies come from. They might just want to know that they are in mommy's tummy.