David Morgan, NUT divisional secretary for Bournemouth, said the impact of what had happened at Portchester school, on his patch, went further than any single school.
It was here that William Gibson worked as a supply teacher, despite having been convicted of indecent assault on a 15-year-old girl in 1980.
Mr Morgan said: "I think it was a complete surprise to everybody. As a teacher, my reaction is the Government is at fault here because sex offenders should not be able to work in schools.
"The teaching profession feels let down by the government department because it has allowed this to happen.
"People had assumed once you were on this list 99 you were not allowed to teach, now we are discovering there are people on the list who can teach.
It makes a mockery of it."
Tony Mulgrew is NUT divisional secretary for Norfolk, where Hewett school employed Paul Reeve who had been cautioned by police for accessing banned pictures of children on the internet. Reeve had been placed on the sex offenders' register but cleared to carry on teaching by Kim Howells, a former education minister.
"I hope a resolution is found because it is not an issue for that school alone, it has become a national issue. I think at present there is uncertainty that needs to be clarified about who should be working in schools," said Mr Mulgrew.