JOHANNESBURG, South Africa &#8212 Oprah Winfrey said Monday she wept for half-an-hour when she heard a dorm matron was accused of abusing students at her school for disadvantaged South African girls. She promised to “clean house” starting with the headmistress.

Winfrey said officials at the Oprah Winfrey Leadership Academy for Girls hid facts and told students to “put on happy faces” and not complain to her. Although she said she was not responsible for hiring at the school, she said the screening process was inadequate and “the buck always stops with me.”

She spoke to reporters in South Africa by satellite from the United States hours after the accused dormitory matron appeared in court near Johannesburg.

Tiny Virginia Makopo, 27, was not asked to submit a formal plea. But she said she was “not guilty” of the 13 charges of indecent assault, assault and criminal injury against six students ages 13 to 15 and a 23-year-old fellow dormitory matron.

Makopo, who was arrested Thursday, was freed on $450 bail and ordered to return to court on Dec. 13.

Superintendent Andre Neethling, from the police’s sexual offenses and child protection unit, said there were at least three serious cases of indecent assault and that the alleged abuse had taken place over four months.

The $40 million school opened with much fanfare in January with a ceremony attended by a cast of celebrities including Nelson Mandela, Spike Lee, Sidney Poitier, Mariah Carey and Tina Turner.

Before the allegations, Winfrey said she had told the pupils she was the “momma bear” who would protect them.

Winfrey said she had been informed by the school’s chief executive, John Samuel, in early October that 15 girls had produced a list of complaints including the sexual assault of a classmate.

“When I first heard about it I spent about a half hour crying, moving from room to room in my house,” Winfrey told the news conference, calling it “one of most devastating experiences of my life.”

The talk show host has spoken in the past of being raped by a distant cousin at age 9 and then abused by three other men, trusted family friends. She has campaigned for laws in the United States to protect children from abusers

Winfrey said that because of the high rates of rape and sexual abuse in South Africa, she had worked to ensure outsiders would not be able to reach students at the school.

But “as often is the case, child abuse, sexual abuse, happens right within the family, right within the confines of people you know,” she said. “My experience with child predators is that no one ever abuses just one child,” Winfrey said.

After hearing of the allegations, she organized an independent investigation headed by Richard Farley, a Chicago detective who specializes in child abuse.

Winfrey said she flew to South Africa to speak with the students and encourage them to come forward with their complaints.

“This was a chance for them to break the silence,” she said.

Makopo was suspended from her job at the school on Oct. 17.