A line of children walk in pairs on a set route, with an adult "driver" at the front and a "conductor" at the back, and there are scheduled stops where children get on and off.
At least 20 "buses" now operate in Scotland with 10 more in the pipeline, says Sustrans Scotland's safe routes to school officer, Philippa Davison.
"This is a great way to get children to walk together safely," she says.
"It provides all the benefits of walking to school. It's good for health and the environment, and it's sociable as children can walk to school with friends and practise road safety skills."
Congestion at the school gate first prompted the idea. "The gates are at the end of a cul de sac that involved parents doing three-point turns outside school," says parent and safety group member Kathrin Simpson.
An assembly and parents' night were organised and pupils' posters advertising the bus were displayed around the village.
Some 12 of Dirleton's 53 pupils use the bus regularly, with an adult driver and conductor at the front and back and some older pupils in between, acting as "junior conductors". No dogs are allowed, bicycles must be walked and footballs carried in bags.
Walking the single village pavement with pupils in their bright tabards (paid for by a local business), it is clear that the bus makes passing drivers aware. It also tends to slow them down, and even elicits smiles and waves.
"The bus is sustained and maintained by the parents," says head Pam Hamilton. "It's a triumph for a school with a small parent base and it builds up rapport between the children and between pupils and parents. It's about citizenship and community."
Those involved range from P1 to P7, with no sense among the older ones that it's a "baby" thing to do. The first bus in May 2001 was welcomed into school with a special healthy breakfast party, and each pupil was given a sticker and certificate. Only one bus has ever had to be cancelled - on account of the weather - and the school has received a special commendation in the Steps to Safety charter which recognises good practice.