unions have warned.

Chris Keates, general secretary of the NASUWT teachers’ union, said the furore could set back its campaigns to reduce malicious allegations. “These recent cases could bring back the imbalance to the system,” she said.

“Teachers, could be condemned where there’s no evidence.”

The union has said that more than 12 criminal allegations are made by pupils against its members each month but only 4 per cent result in a conviction.

John Dunford, general secretary of the Association for School and College Leaders, said that there had been an “unpleasant witch-hunt” against Ruth Kelly.

“It’s unhelpful to the real issue, which is how heads can best be helped to ensure that children are not in any danger,” he said.

William Glover, a housemaster at a private boarding school in Surrey, said he was concerned about the impact the furore could have on offenders.

He represents the public on the strategic management board for public protection in Sussex, which oversees agencies monitoring sexual or violent criminals who have been released. “In the long-term this is going to do great damage,” he said. “Offenders will fear the same persecution, so will go underground.”

Opinion was divided on The TES online staffroom about whether sex offenders should automatically be banned from school. Some called for complete bans.

But others said people had been put on the sex offenders register, seemingly unfairly, for crimes such as sleeping with a 15-year-old boyfriend or girlfriend when they were 16.