More than 500 male students and staff at a French school will undergo DNA tests from on Monday in a bid to discover who raped a 16-year-old girl in a dark school toilet.

The tests, a first in a French school, will end on Wednesday and target 475 high school students, 31 teachers and 21 others present on the premises on September 30 when the rape happened.

The schoolgirl was assaulted in the private Catholic Fenelon-Notre-Dame high school in the southwestern Atlantic port city of La Rochelle.

The attack took place after the light from an automatic time switch went off and she therefore could not give the physical details of her attacker.

There are a total of 1,200 students in the school.

The arrival of 16 officers at the school on Monday has naturally affected the atmosphere among the pupils.

“It remains tense, because this is the first time we have to do DNA tests, so it seems quite unique to us,” one pupil told Europe1 radio. “It’s disturbing to have to do the test, It’s bizarre.”

Others talked about the fear of finding out who the attacker was.

"We want to know [who it is], but deep down, we don't, because what if it is a very close friend?" one schoolgirl told Europe1. "We would not be able to live with that. It is monstrous what he did to this girl, it's horrible."

The cost of the operation will be around €5,000 ($6,940) and saliva swabs will be taken and matched with DNA found on the girl's clothes.

DNA from her clothes had tested negatively when matched with those of her family and close friends. The results are due out in a month.

Isabelle Pagenelle, the prosecutor of La Rochelle, said both parental and individual authorisation was necessary for minors undertaking the test.

She said there would be no forced DNA testing but added that "those saying no can become potential suspects who may be detained." Authorities said samples that do not match the DNA found on the victim will be destroyed.

Results not connected with the rape will be destroyed, Pagenelle said, adding that the tests were necessary to prevent further assaults and make the school safer.

The school's principal Chantal Devaux said the rape had hitherto remained a secret and only investigators were in the loop.

The samples will be drawn using a swab under the tongue and the results of the tests are due in several months.
  
Prosecutors said they had decided to go ahead with the mass DNA tests after several months of investigation proved fruitless.

But some have condemned the move as a clear violation of civil rights.

"Refusing to give a DNA sample when not in custody is a right," prominent defence lawyer Joseph Cohen-Sabban told newspaper Le Figaro.

"It's ludicrous! They want to decide on taking someone into custody based on that person exercising their rights," he said. "Then, once in custody, it's against the law to refuse to give a DNA sample… This is a truly unacceptable abuse of process."