A school has been accused of failing to protect a pupil who said she was raped twice by students.
Stanbridge Earls School for children with special educational needs in Romsey, Hampshire, was found to have discriminated against the girl by excluding her following the allegation.
A tribunal said the school found she had breached rules prohibiting engagement in sexual activity.
The tribunal said it had cause for "grave concerns" about safeguarding.
The girl, who cannot be identified for legal reasons, no longer attends the boarding school.
The First-tier Tribunal Special Educational Needs and Disability found the school failed to protect a vulnerable disabled pupil who was a victim of grooming and sexual abuse by male pupils at the school.
It branded the failure "inexcusable" and said serious concerns need to be addressed by the authorities.
The tribunal said head teacher Peter Trythall's conduct "borders on contempt for statutory duties".
In a letter to parents, Mr Trythall said they took the tribunal decision "extremely seriously" and would rectify the shortcomings identified.
He wrote: "This was a complicated and distressing case and the first of its kind ever experienced by the school, and is not representative of the way we normally meet the needs of our pupils.
"We are writing personally to the pupil concerned to apologise."
The tribunal heard the girl visited the school nurse and said she had had a sexual encounter.
It said the school did not contact the girl's parents, believing she had consented to the encounter.
She later told her mother that she had had sex in a separate incident and she contacted the school.
The claim was reported to the police via Hampshire Social Services when her parents were told about the previous sexual encounter.
The following term the girl says she was raped again and the school said she would have to be excluded because she had broken rules by having sex on school grounds.
Speaking to the BBC, the girl's father called for some staff to be removed from the school to protect pupils.
He said: "There is an enormous amount of retraining there to protect others and I think there's also some staff removal needed to safeguard the people that are there."
Hampshire Constabulary said it thoroughly investigated the two sexual assault allegations before passing a file to Crown Prosecution Service (CPS) which decided not to prosecute.
The girl's parents appealed against the decision but it was upheld following two reviews.
A CPS spokesman said in a statement there was "insufficient evidence" to provide a realistic prospect of conviction for rape.
It added: "The CPS was satisfied that there was sufficient evidence to prove that offences of unlawful sexual activity with a child had taken place, but that it was not in the public interest to prosecute the two boys concerned."