He is aware of the problems facing schools under pressure to save money on transport. "But it means there are old coaches running around with rusted frames. If seatbelts connected by rivets to a rusty panel give way in a crash, there is enormous pressure on the child - like a cork popping out of a bottle," he said.
Mr Lock is calling for solid anchor points for seatbelts to be compulsory on old coaches. If necessary, operators should be forced to strengthen coaches by welding in new metal panels.
Stroud Labour MP David Drew has also tabled a Commons question demanding to know what has been done to improve safety on coaches carrying young people.
From February, coaches without seatbelts will fail their MOTs. Installation will be checked in the first MOT test after seatbelts have been fitted.
Ivan Meredith, a coach operator in Kidderminster who has advised David Lock, lost all his school contracts after being undercut by rivals with older vehicles. He recently bought an old bus that had been in regular use but which had a corroded subframe. He uses it to show parents and headteachers the risks with elderly vehicles.
Following lobbying from the transport safety pressure group BUSK (Belt Up Schoolkids), Longney C of E primary school in Gloucestershire now only uses coaches built after 1993 - with strengthened shells - on trips of more than 50 miles.