Please Sir! (1971, John Alderton, Derek Guyler)

Jaunty film spinoff from the TV series opens with teacher Mr Hedges (Alderton) catching a London Transport AEC Routemaster on service 8 to get to school. He and his class go on a school trip - transport is provided by two Harrington Cavalier bodied AEC Reliances of Jack Crump, BMK341A and GMV514B.
BMK's driver is played by Jack Smethurst, seen not quite running down caretaker Mr Potter (Derrick Guyler)
The Fenn Street School kids get to travel on BMK341A
(thanks Adam Floyd and Jon Price)