Harrow School is standing by its art teacher after photographs of her posing topless were circulated among pupils.
Joanne Salley, a former model and TV presenter, was photographed in at least two provocative shots wearing just a pair of light-blue jeans.
Headteacher Barnaby Lenon said: "Joanne Salley is a long-standing and successful art teacher at Harrow who has, in the past, been a model.
"Photographs of her modelling were taken by a professional photographer who also teaches photography part-time at the school.
"These pictures were stolen and passed on to some pupils at the school without the agreement of the photographer, who holds the copyright. The matter is being investigated."
The pictures were taken by Fiona Corthine, within the grounds of the school where she and Miss Salley both live, in what a school spokeswoman suggested was an "art project". She said the pictures were stolen by a pupil, who is thought to have found them on a memory stick in the photography laboratory.
The school spokeswoman dismissed as "hearsay" reports that the majority of the school's pupils, teachers and kitchen staff had seen the photos.
She also said reports that Miss Salley had left the school in a flood of tears were "definitely not true", adding: "She hasn't left the school. She's a very good art teacher."
She was unable to confirm Miss Salley's whereabouts on Friday, adding: "They're on end-of-term timetable. I have no idea whether she is teaching or not - she may not have had any classes in her timetable but if she's supposed to teach, she's probably teaching."
Harrow School, founded in 1572, has male pupils aged from 13-18.