An ITV News investigation has found almost one in five school buses - checked by safety officials - had defects, and the number is increasing year on year.
Across England, Scotland and Wales around five vehicles a week are being identified with problems - some of them serious.
What's more, an industry whistle-blower has brought us proof that some buses were even carrying pupils without an MOT.
I joined an early morning operation in the West Country as unannounced spot checks were made on school buses by the Driver and Vehicle Standards Agency (DVSA).
It was a worthwhile operation, the team identified one coach with a braking defect.
Yet we've found that the number of these spot checks fell dramatically last year – to about half the normal level. Officials say this was due to "pressure on resources".
ITV News has obtained new unpublished figures from the DVSA showing:
1,439 spot checks were made on school buses in the last year – a fraction of the estimated 60,000 school buses.
Almost one in five (18%) were found to have faults, a 5% increase on last year.
And 121 defects were considered serious. These included "excessively worn brake pad", "tyre damage" and "uninsured" vehicles.
The industry says operators do their own "thorough checks" as well as an annual MOT, more detailed than for cars.
But an industry whistle-blower told ITV News some school buses were on the road without a MOT.
Three school buses in Camden, North London illegally carried pupils – some for almost three months.
The council says it was a "recording error" but not unsafe because they do regular in-house safety checks. They have apologised and say they have made changes.
When you put a child on a school bus it's an act of trust. Our findings show how important it is to check on safety – and that it cannot be taken for granted.