Under certain circumstances drivers without a full category D1 entitlement may drive a minibus within the UK provided that it is not being used for hire or reward - ie provided there’s no payment from or on behalf of the passengers.
Those who passed their driving test before 1 January 1997 automatically received category D1(101) (not for hire or reward) entitlement, so school staff with such a licence can drive a minibus carrying up to 16 passengers with no maximum weight limit,
Those who passed a driving test on or after 1 January 1997 may drive a minibus with a maximum unladen weight not exceeding 3.5 tonnes and not for hire and reward under certain conditions outlined in the document.
Non profit making organisations including schools may provide transport for their own members or for the local community on a hire and reward basis under a section 19 permit.