Prohibition notices were served on the vehicle operators outside Ysgol Glan Clwyd in St Asaph and Ysgol Emrys ap Iwan at Abergele on Monday.
Police and staff from vehicle agency VOSA carried out the checks when drivers ferried pupils to school.
Chief Insp Darren Wareing said all the region's counties will be checked.
Three buses and an eight-person taxi were given prohibition notices at St Asaph - three for illegal tyres and one for a coach with a loose external panel.
A further three notices involving illegal tyres were served on buses arriving at Ysgol Emrys ap Iwan as part of the road safety initiative Operation Coachman.
The operators were instructed to have work carried out before the vehicles would be allowed back on the road.
Mr Wareing said: "We are there to enforce the law and ensure kids have a safe journey to and from school.
"We have received co-operation from drivers, operators and schools, but anyone not complying with the prohibitions could then face prosecution."
Six school buses and a taxi on a school run have been ordered off the road as part of a safety crackdown by North Wales Police.