David Morgan, NUT divisional secretary for Bournemouth, said the impact of what had happened to 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 for indecent assault of 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."
He said he did not think most teachers would feel their lives had been made more difficult, but supply teachers might feel more vulnerable to suspicion.
Mr Morgan, a former deputy head still does some supply work himself. He said: "Perhaps supply teachers should carry their certificate around with them. I will be waving my clearance certificate around."
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.
He said that the Norwich school was now dealing with the repercussions of the case. "The teachers have a job to do of getting on with teaching and doing their best for children. The life of the school must go on.
"The bigger picture is being tackled elsewhere. I hope a resolution is found because it is not an issue for that school alone, it has become a national issue," he said.
"I think at present there is uncertainty that needs to be clarified about who should be working in schools. There was uncertainty, there was a lack of clarity. There has to be a resolution."
Contributors to The TES website also called for greater clarity and consistency in the rules. "chili pepper" said: "Currently I am only allowed to employ staff with enhanced clearance. If someone else can override this, why have it as a rule?"
Alison King, who chairs the Local Government Association's children's committee, said local authorities had had difficulties referring concerns about individual teachers to the DfES' safeguarding children unit.
"They often have to wait for months before there is any action and even then the unit might not inform them about it," she said.
"The whole thing seems to be a muddle. The process needs to be reviewed so there are not so many different lists."