Over 40 schools have been warned about employing teachers who have not fully completed criminal records checks.
All school staff, including teachers, have to complete pre-employment vetting before they can work with children.
Teachers are required to have an "enhanced" check, which discloses things like previous convictions.
But 45 schools have been warned that they have employed substitute teachers whose pre-employment checks were not complete.
The names of the schools have not been revealed.
