IT could be argued that William Gibson made a dreadful mistake more than 25 years ago when he began an affair with a 15-year-old pupil who attended the North-East school where he taught.

Should he be harangued for the rest of his life for that one error of judgement?

Perhaps if Mr Gibson's case were a one-off - and perhaps if he did not have a conviction for £60,000 fraud in Durham as recently as 2000 - the Department of Education would be able to get away with it.

But a pattern is beginning to emerge in the department's behaviour. A man who receives a police caution for downloading child porn is allowed to teach PE in Norwich - PE, which involves children undressing, for heaven's sake.

Another man who compiled a scrapbook of pornographic pictures of young boys is allowed by the department to teach - at an all girls school, as he is deemed as no risk to the pupils there. But is he really the sort of role model we want educating young minds of either gender?

And let's return to the letter sent out by the department to Mr Gibson on Secretary of State Ruth Kelly's behalf. It tells him: "You are warned that any further misconduct on your part, which calls for action by her (Ms Kelly), is likely to have more serious consequences for your future career as a worker with children."

So the department is placing him in charge of children, but lacks so much confidence in him that it feels it is necessary to warn him to keep his hands off.

Yet, unless Ms Kelly's fingerprints can be found on this letter, it appears that she was not personally involved in these three glaringly incorrect cases. She will probably cling to her job - even though her department has not implemented Sir Michael Bichard's Soham inquiry recommendations, which warned that the way offenders were appointed to jobs in which they had contact with children was in a mess.

However, she has used up all of her political capital. From this diminished position, she will struggle to pilot the controversial Education Bill through the Commons against her backbenchers' wishes.