A private Catholic school in France is to test the DNA of all male pupils and staff in an attempt to identify the rapist of a 16-year old girl who was assaulted in the school toilets.

In the first time this mass screening has been carried out in a French school, 527 pupils and staff will be genetically tested from Monday at the Fénelon Notre-Dame school in La Rochelle.

The victim said she was unable to identify the rapist, who attacked her from behind in darkness after the light on an automatic time switch went off, according to the prosecutor Isabelle Pagenelle. But traces of DNA were found on her clothing.

Those to be tested include 475 pupils, 31 staff and 21 other employees who were in the school in western France at the time of the attack on t30 September last year.

The DNA is to be collected in saliva swabs over three days by 18 police officers who will be at the school from Monday. Pagenelle told a news conference that the DNA from the girl's clothes had tested negatively against the victim's family and friends.

But the genetic testing of teenagers has proved controversial, particularly as the authorities say that anyone who refuses to take part will be considered a suspect.

"I understand the argument that those who have done nothing wrong have no reason to refuse. But why should refusal be considered an admission of guilt?" said Pierre Tartakowsky, president of the French human rights league. He told Sunday's edition of Le Parisien newspaper that the testing was "disproportionate, threatening and traumatising".

"The issue is to know whether the sole fact of being a pupil at this establishment is a plausible enough reason to become a suspect," said Françoise Martres, the head of the French magistrates' union.

Pagenelle said there was a significant probability that the attacker was from the school. Police had a list of all those on the premises at the time of the rape, she said. The school, in the centre of La Rochelle, has 1,300 pupils.

The headteacher, Chantal Devaux, who informed pupils and their parents on Friday about the testing, said: "We have followed every lead before going down this route. We don't have any other choice." Testing of minors at the school will require parental approval.

The genetic-testing results will be known within a month. Pagenelle said all results unconnected to the rape would be destroyed.