Paris - Male students and staff at a high school in western France - 527 people in total - were asked to give DNA samples beginning on Monday as police search for the assailant who raped a teenage girl.
The La Rochelle prosecutor's office said tests were expected to last from Monday through Wednesday at Fenelon-Notre Dame high school, where the 16-year-old was sexually assaulted on 30 September in a dark bathroom.
Police recovered genetic material from the girl's clothing, but it had no matches in the national DNA database.
"This happened during the school day in a confined space," Chantal Devaux, the private Roman Catholic school's director, said in interviews with French media.
"The decision to take such a large sample was made because it was the only way to advance the investigation."
Summonses went out last week to 475 students, 31 teachers and 21 others - either staff or males who were on campus at the time. The cost to analyse the samples is estimated to total €5 000.
Devaux acknowledged that all the results could come back negative, sending investigators back to the drawing board.
France has an extensive DNA database, with a total of 2 million profiles kept on file as of 2012, and refusing a test can be used against suspects in court.