William Gibson, 59, was convicted of indecent assault on a child in 1980, but found work as a supply teacher in South Tyneside during 2003 and 2004.
Gibson, who was jailed for fraud in 2000, also taught in County Durham.
The government ordered an inquiry into sex offenders in schools following a similar case in Norwich.
In that incident, former education minister Kim Howells approved the appointment of a PE teacher who had been cautioned for accessing child pornography on the internet.
Opposition MPs and school groups have criticised Education Secretary Ruth Kelly over the incidents.
Shadow schools minister Nick Gibb said Mr Gibson’s case raised more concerns about the measures in place to protect pupils.
“It will undermine confidence that parents have in the system that’s meant to be there to protect their children from this kind of thing.”