BUS routes to Sedbury's Wyedean School are facing disruption this week as the school tries to pull out of a transport contract with St Arvan's based J & D Langley Coaches.
The school is understood to have had increasing numbers of complaints from parents about late buses and poor service.
In a letter to parents sent out last week assistant head Martin Jenkins said two of the three vehicles operated by Langleys were recently given prohibition orders and taken off the road.
In a statement from the school Mr Jenkins said: "We have been increasingly concerned about the service Langley's Coaches have been offering to our parents who are outside our traditional catchment area.
"The service is an independent contract between parents and Langley's but on advice from the Integrated Transport Unit at Gloucestershire County Council we feel we have no confidence in Langley's providing a service for our students.
"All Gloucestershire County Council contracts have been withdrawn from Langley's for a number of reasons and as a school we felt it important to inform parents of our concerns.
"We have been working with the Integrated Transport Unit to offer our parents the opportunity to work with another bus operator as an alternative to Langley's.
"Hopefully this service will commence in the next couple of weeks."
Parent John Parker, of Lydney, said he was concerned that a lot of children had been late for their mock exams.
His son Tom, 17, is a sixth form pupil and has now started catching the number 73 Stagecoach bus to school.
Mr Parker said: "I have noticed a lot of children seem to be catching the number 73 at the moment and no one seems to be waiting for the Langley's coach. Obviously we are not happy about this as parents are paying for a service we're clearly not having."
A spokesman for Langley Coaches said it was a complicated situation and they would be contesting the school's decision.
He said: "Basically we can't comment at the moment – it is going to be put in the hands of the solicitors. There have been problems – no one's denying that, but we're working flat out to rectify matters. We have had the backing of a lot of parents."