There were 307 buses found to be dangerously defective in the past year.
The Chief Executive of the Road Safety Authority said the figure was "disquieting" and it was evidence that there are a number of unscrupulous operators who are failing to continually carry out preventative maintenance on their vehicles.
Moyagh Murdock was before the Oireachtas Transport Committee to discuss school bus safety after figures published by RTÉ's This Week showed high numbers of buses were failing road side safety checks.
Ms Murdock told the committee that over 14,000 buses were tested as part of the testing regime for commercial vehicles between July 2017 and June 2018.
The average pass rate was 59%, but she said it was "still disquieting that 307 buses were identified as being so defective that they were deemed to be dangerously defective".
"The older the bus the higher the fail rate, which shows the importance of ensuring there is an adequate and preventative regime in respect of school buses" she said.
"It is evidence from the number of dangerously defective buses detected at the annual test, that there are a number of unscrupulous operators who are failing to continually carry out preventative maintenance.
"This is a systematic failure by those owners concerned.
"It's an unacceptable practice and clearly a blatant disregard of road traffic law and also a failure to appreciate the potentially adverse road safety effects for the general public using such a vehicle."
Ms Murdock said operators have "a duty to ensure that their vehicles are road worthy at all times, not just during the annual test."
Asked by Fianna Fáil TD Robert Troy why the RSA refused to share its index of the riskiest bus operators with Bus Éireann, which tenders for school bus contracts, Ms Murdock said this was personal data.
The committee was told the RSA was legally advised that it could not share this information, and it would be "dangerous" to disclose the information for data protection and commercial reasons