My thoughts on this specific role:
Hours wont be great - Im assuming 8am for an hour or so and again at 3pm for another hour or so. Thats often when parents/grandparents etc are doing their own school drop off
Driving licence - most people dont have a licence that allows them to drive a minibus. I passed in 2000 and dont have D1, my DH passed in 1993 and does. Anyone 40's or below is unlikely to have D1 on their licence unless they took a separate test.
Often people who work part time want flexibility to fit it around their other responsibilities, but this role has none as it cant be done at home or in hours that suit the applicant. This means that people who do want to work part time for the money offered will find employment that does this rather than being tied into the requirement of this role.
Its the sort of role that would ideally suit somone who is older, possibly retired, thats wants to top of their income. I actually know 2 people who do this role - both of them are over 70 and do it to top up their pensions and I think they found out about the roles through word of mouth as they are not unemployed, on benefits or actively looking for work so dont check out the usual places you probably advertised.
If you could try to get the role targeted to this type of person you may get more applicants.
Could you advertise it in your school newsletter and ask parents to pass it onto to anyone who may be interested? What about the local newsletter (we get a couple here delivered free with ads for local businesses, whats on in the area etc) as that would widen your applicants too?
Good luck