TimePlan employed 26-year-old Canadian Gehring, who was cleared of indecently assaulting two boys attending the Surrey school where she worked. She later admitted to having had sex with a 16-year-old boy at another school.
The Criminal Records Bureau was set up in March to carry out police checks on people working with children or vulnerable adults. But because of delays, a fast-track procedure has been introduced. This allows teachers to be employed once a check on the Department for Education and Skills' List 99 has been done. A full check on other convictions is carried out later.
Ms Gehring did not have a police record and would not have been picked up by the bureau's checks. But the affair has left TimePlan anxious to minimise the risk of employing rogue staff in the future.
TimePlan spokesman Barry Hugill said: "Having gone through the Amy Gehring case there is no way we would risk anything like that again.
"Putting teachers into schools without police checks seems very worrying because it is not fair on schools and parents. We're refusing to do that."
Despite the fast-track procedure, complaints about delays at the bureau continue. The parliamentary ombudsman, who investigates claims of injustice by public bodies, is looking into three complaints about delays.
The agency Supply Desk has threatened to take legal action against the bureau saying that the delays in checks are creating "enormous losses" for the company. It says 280 of its teachers who applied before July 1 are still waiting for clearance.
However, a bureau spokesman insisted that all applications from teachers received before July 18 have now got at least a List 99 clearance.
He said: "We have issued in excess of 25,000 List 99 checks. If there is a nominal amount of checks that have slipped through the net we will want to rectify that."
Checks are taking up to six weeks, the target time is three weeks.
Supply Desk managing director Steve Petherbridge said: "Our experience of the Criminal Records Bureau is chaotic.
"The bureau is charging us for a service that is appalling and we just have to pay because there is nowhere else to go."