Schools in England are to be given "crystal clear" guidelines to prevent sex offenders being employed, the education secretary has announced.
Schools will have to show records of checks carried out
Alan Johnson said staff vetting would be tightened and that inspectors would check schools' record keeping.
If this is not satisfactory a full Criminal Records Bureau search will take place, he added.
Mr Johnson's comments follow an Ofsted report criticising schools for failing to keep records of staff vetting.
Inspectors found there was a culture of thinking it was "somebody else's job".
Most schools showed "good practice" in making staff checks, yet took supply teachers from agencies "on trust".
Mr Johnson said his department would publish revised guidance stating that every school must collate records.
He told the Commons: "We need eternal vigilance to protect children from those whose nature it is to manipulate and deceive.
"And it's our responsibility to make sure the system is as robust as possible and that any failings are addressed as soon as they are identified."
He added: "We will act quickly but carefully to tackle the failings that Ofsted has identified."
The Ofsted report was commissioned in January by the then education secretary, Ruth Kelly, amid a row over dozens of sex offenders being cleared by ministers to work with children.
Mr Johnson told MPs that all but one of the people kept off List 99 - of people barred from schools - had been re-assessed by police as posing no threat.
The other was not working with children and had not been identified as an "immediate cause for concern".
There were also previously three outstanding cases of people on the sex offenders' register, but who had been cleared to work in schools.
Mr Johnson said two had been reassessed as not posing a risk. The other was still being investigated, but was not working with children.
Ofsted inspectors, who looked at 58 schools, found "hardly any" took "even the simplest of measures in terms of record-keeping".
Most did not note whether staff were who they claimed to be, had the qualifications they claimed, or had a criminal record.
Among the "unacceptable practices" found, only eight of the schools had asked supply agencies for evidence that checks had been made.
This could "compromise the way children are safeguarded".
Schools in England have to gather and collate "specific evidence" of whether potential staff pose a threat.
Ofsted found all the schools thought the local authority was carrying out Criminal Records Bureau and List 99 checks, but most did "not know".
Maurice Smith, chief inspector of England's schools, said: "Currently, everybody thinks that somebody else is doing this, or that it is somebody else's job when, in fact, hardly anybody has secure evidence that any of the simplest tasks have been completed at all, or when."
Shadow education secretary David Willetts said it was "a truly shocking state of affairs".
Meanwhile, Liberal Democrat education spokeswoman Sarah Teather called it "extremely worrying".