About 20 psychiatrists and psychologists from federal hospitals have begun interviews with the girls who are still sick or have been sick, he said, adding that he hoped to have a preliminary report by the end of the month.

When the scale of the illness became clear, the school allowed parents to take their daughters out for a week. The sick ones promptly recovered, and they have been trickling back to school, somewhat to the surprise of reporters and television crews outside the gates who have crowded the girls and their families to look for evidence of mistreatment inside.

"We need support, not scandal," Cheong said in an interview at the school. She seemed exhausted by the media attention and clearly concerned that it might scare away some of the donations she needed for the school's $4 million operating budget.

Founded in 1990, the school is one of 10 in Asia and Latin America operated by a charity called World Villages for Children. It is run by nuns from the Sisters of Mary, an order founded in South Korea in 1964 by an American priest, Monsignor Aloysius Schwartz.

The school here offers three years of middle school and two years of technical high school to girls from 12 to 17.

The expansive campus is at the dust-veiled edge of Chalco, this vast working-class suburb of Mexico City. With a population of as many as half a million, Chalco is a watchword in Mexico for urban poverty.

An unplanned settlement of concrete-block houses that the owners can never afford to finish, Chalco has mushroomed over the past 30 years as migrants have moved from the countryside to look for work in the capital.