My DCs have been using Public Transport from day 1, and that's after always having me take them to Primary School, never going alone. I did the journey with each of them a couple of times before they started (very boring!!). They now pride themselves on their extensive knowledge of the West Midlands bus network! Occasionally I give them a lift, but with 3 children at 3 different schools it's not often practical, so lifts are saved for the neediest, ie. late home or heavy bag! There are children on the bus from different schools and they seem to mix well (it wasn't like that when I was at school
I guess it depends where they have to get to, and what sort of roads they have to cross. The Green Bus takes them right to the school gates. I have no direct experience of the Green Bus, but I have never heard that it is unreliable, and at a talk at KEA a couple of years ago, the Green Bus rep stated that they work their routes around the needs of the children who use it, modifying the route slightly each year as necessary to ensure a local pick up point for everyone.
Personally I think that if it is possible for children to safely use any bus or the train, it is an enriching experience to travel with friends rather than their parents to and from school. They make friends with children in other years, and it's always handy to have some older friends! Having said that, I did take each of them for the first week or two, simply because I thought they had enough new things to learn without worrying about getting to and from school and I will do the same with DC3.