The discovery was made during a five-day operation between June 18 and 22 by Central Scotland Police and the Vehicle and Operator Services Agency (VOSA) at Blair Drummond Safari Park, near Stirling.
The most serious defect was an emergency door which could not be secured shut. Officers also discovered braking defects, a faulty exhaust, oil leaks and a broken seat belt.
Sergeant Bob Murphy said: "It is extremely worrying that nine buses were so defective they had to be removed from the road.
"We want to reassure parents we will do everything we can, along with VOSA, to make sure their children are travelling on buses that are roadworthy."
During the operation, 15 drivers were also found to be working in excess of their hours or unable to prove the hours they had worked, and were not allowed to drive until they had had a sufficient rest period.