A review of school transport contracts by the Scottish Consumer Council found "unacceptable variations" in the quality of the service generally, with a lack of disclosure checks potentially the most serious flaw. It has made 14 recommendations for improvement to the Scottish Executive.
The survey, in which 27 of the 32 authorities took part, found that only 37 per cent carried out "enhanced" checks on drivers with Disclosure Scotland.
While 78 per cent said they undertook Disclosure Scotland checks, they did not state whether these were "basic", "standard" or "enhanced". Three authorities did not say if they carried out checks, another said it would "soon be undertaking enhanced disclosure checks" and one checked out only taxi drivers, which the SCC deemed "unacceptable".
Existing guidance from the Executive does not specify which level of check should be carried out on bus drivers and escorts, simply stating that it expects they will be "fully scrutinised" by Disclosure Scotland.
The SCC report recommends greater consistency among councils and enhanced checks on drivers and attendants before school transport contracts are awarded.
The survey also found inconsistencies in the use of attendants on buses.
More than half of authorities never use them, yet 45 per cent of them believe behaviour is getting worse. This is despite the Executive's guidance that supervision on transport can nip misbehaviour in the bud and stop it spilling over into the classroom.
Graeme Millar, the SCC chairman, commented: "The quality of school transport is in danger of being determined by local authority boundaries.
If all pupils are to have a safe and reliable school transport service, which will encourage them to use public transport beyond their schooldays, and give confidence to parents, then this is not acceptable."
One positive finding was that 90 per cent of authorities go beyond the statutory minimum of free travel for under-eights who live two or more miles from school and for over-eights whose homes are three or more miles away.