The teenager attacked the 28-year-old as she marked books at the end of her second day. The teacher struggled to defend herself during the ordeal, which continued for 12 minutes at the central London school, but the boy, 15 at the time, head-butted and threatened to kill her before he overpowered her.

After the hearing it emerged that it is believed the boy was accused of making a sexual threat towards a woman in her twenties in Kennington, south London, in April last year. No one was charged as a result.The boy had denied raping the teacher during a preliminary hearing at the court last month. But yesterday he changed his plea and admitted raping her on 7 September last year. Two other charges alleging sexual assault were ordered to remain on file.

Brendan Kelly, for the prosecution, described the teacher's ordeal as "a sustained and violent and sexual attack". Mr Kelly said the woman had been bitten, head-butted, and indecently assaulted.She had been marking books at the desk in her classroom at the end of the school day when she noticed that the door had swung open. She did not look up, but then felt a strong arm round her.

At first she believed it was a student joking. "But she felt the grip tighten and she was pulled out of her chair," Mr Kelly told the court. "He was telling her to be quiet and dragged her the length of the classroom. He is a large boy. He had no trouble dragging her, while telling her to be quiet."

The teacher - who said teaching was "very much her vocation" - tried to fend the teenager off by stabbing him with a pen. She dropped it, but continued to struggle. Mr Kelly said:"As she tried to push him away, he grabbed her waistband and ripped her trousers from her with one movement. The attack continued."

After the attack the teacher ran downstairs - blood pouring from her nose and down her back - to the headteacher's office. Those present were unable to recognise her because her face was covered in blood.When arrested, the teenager tried to explain away blood on his trousers, saying he had been attacked at a south-east London train station. But the blood was shown to have clearly come from the teacher.

The youth - who cannot be identified because of his age - had a previous conviction for being in possession of a knife and had been placed on a community punishment order. He was remanded in custody for reports and will be sentenced on 17 June.Police said the youth had not met the teacher before the attack and was not in any of her classes. Closed-circuit television footage showed him wandering round the school looking into classrooms before making the attack.

The court was told that the teacher had still not returned to work. A statement from her, which was read outside court, said: "It has been a gruelling wait for me and my family to see that justice has been done and this individual is prevented from harming anyone else."Detective Chief Inspector Andy Rowell, of Operation Sapphire, New Scotland Yard's sex-crime investigation unit, described the attack as a "particularly violent sexual assault".

Det Ch Insp Rowell said: "It is extremely rare and unheard of for an attack to happen in a school in such a way with such a level of violence. The school had an exemplary record. It is not one which would be considered to be a catalyst for this type of behaviour."Det Ch Insp Rowell added: "The victim in this matter has shown immense courage and bravery throughout the ordeal and during the police investigation. She now wishes to embark upon a period of rebuilding her life."

By Sarah Cassidy, Education Correspondent
The Independent 4 May 2005 13:06