I have successfully utilized remote keyless entry on multiple buses. The latest one i used was just a standard part sold at orielly auto. It was a standard universal remote keyless unit made for cars that already have electronic locks. The thing that made it work for me is that it had an option to provide current for 3 (or 5? i forget) seconds in order to give older European vacuum operated locks time to work. This was about the amount of time it took to open/close the door on the bus.
Bulldog Security - Keylees Entry
Part Number: KE1702
Meijer sells this item for $36.00.
Another option i considered, but never implemented, is using a magnet switch made for windows like they use in a burglary alarm system. You could mount the part without the magnet somewhere on the exterior of the bus, and you could give a magnet to each person who you wish to grant access to the bus. They can pass their "magic" necklace, ring, card, key etc over the correct spot on the bus and the door will open. You don't have to tell people that their magic key is a magnet. Obviously this system is not the most secure, as anyone with a magnet could gain entry to the bus.