A headteacher has admitted an investigation into an incident of bullying against a member of staff had proved fruitless.

In July, the Oxford Mail reported allegations that a member of staff at Gosford Hill School, in Kidlington, had a photograph of herself in her underwear taken from her mobile phone and distributed to pupils.

Headteacher Dr Stephen Bizley said he feared the woman, who appeared to have taken the photo in a mirror, had been the victim of "a vicious and personal act of bullying".

He launched "a full and immediate investigation" and refused to rule out the possibility the pupils responsible would be expelled.

Thames Valley Police also investigated "allegations of theft".

However, Dr Bizley said last night the school had been unable to resolve the issue, and said those responsible for the alleged offence would not be punished.

In a formal statement, he said: "The incident was fully investigated, but there was no conclusive evidence of what happened.

"Despite many hours of work the school was unable to get to the root of the incident. The matter is therefore closed.

"At a time when we are celebrating the best GCSE results the school has achieved I do not wish our success to be overshadowed by one incident.

"The vast majority of our students behave well and make a major contribution to our community."

When news of the incident emerged, Dr Bizley stressed there was no suggestion the member of staff had been at fault.

He said: "Investigations strongly suggest the staff member concerned was targeted and the subject of a vicious and personal act of bullying."

Meanwhile, the Association of Teachers and Lecturers told the Oxford Mail incidents of 'cyber-bullying' — involving the use of internet sites or mobile phones, in particular — were an increasing problem.

Spokesman Christine Gregory said: "It's a national issue. Increasingly it's happening and schools need to have very robust policies to deal with pupils and other colleagues who bully staff."

Dr Bizley added: "The incident highlights the dangers the education profession faces in the world of internet and mobile phone technology."