Thirteen adults have been barred from working with children in England even though they had previously been cleared to do so, the government has said.
Mr Johnson says pupils' safety is paramount
In a statement to MPs, the Education Secretary, Alan Johnson, said that the action followed a review of their cases as part of a tightening of the rules.
Mr Johnson insisted that protecting vulnerable youngsters from sex offenders was his "top priority".
But opposition MPs said the government was acting too slowly ensure safety.
Twelve of the 13 adults in question had previously been given partial bans and one had been entirely free to work with children.
The 13 individuals are now all on the Department for Education and Skills blacklist known as List 99.
Another 32 people - who were already on the Sex Offenders Register - are also being added to List 99.
"The safety of children and young people is our top priority," Mr Johnson said.
"We are introducing changes to ensure that we have the toughest ever vetting and barring system for all those working with, or seeking to work with, children and vulnerable adults."
The figures come 14 months after Mr Johnson's predecessor Ruth Kelly nearly resigned over disclosures that ministers had cleared sex offenders to work in schools.
It emerged in January 2006 that Ms Kelly's department had cleared a man on the sex offenders register to work as a PE teacher at a school in Norfolk.
After two weeks of intense pressure over the row, Ms Kelly made a statement to the Commons in which she revealed that 88 offenders had not been barred.
New rules which came into force on Wednesday mean any adult convicted or cautioned for a sex offence will be automatically included in List 99.
'Unacceptably long period of time'
Margaret Morrissey, from the National Confederation of Parent Teacher Associations, said: "Parents can't be blamed for worrying if they are not given the full details and told the truth.
"Parents will think: 'I can't trust this, for all I know I might find that the next individual to be barred is the person who has been taking my child for physical education'."
The Shadow Education Secretary, David Willetts, said ministers were moving "far too slowly".
"Ruth Kelly said last January that regulations would be introduced 'shortly' automatically barring those with cautions from working with children," he said.
"Now we learn this is only happening today - nearly 14 months later.
"Parents and teachers need reassurance as swiftly as possible."
The Liberal Democrats' education spokeswoman, Sarah Teather, said: "Have ministers forgotten how much of a scandal this was 12 months ago?
"Parents will be extremely concerned to know that this loophole has still not yet been closed and people are still being vetted.
"There seem to be serious questions about the lack of haste by the department - it's been over a year since these 210 cases came to light.
"This is an unacceptably long period of time for children to be left at potential risk."