Twelve-year-old Suminah is not. She looks like she wants to punch somebody. Under her white hijab, which she has yanked down over her brow like a hoodie, her eyes have the livid, bewildered expression of a child who has been wronged by people she trusted. She sits on a plastic chair, swatting away her mother's efforts to placate her with a party cup of milk and a biscuit. Suminah is in severe pain. An hour earlier, her genitals were mutilated with scissors as she lay on a school desk.

During the morning, 248 Indonesian girls undergo the same ordeal. Suminah is the oldest, the youngest is just five months. It is April 2006 and the occasion is a mass ceremony to perform sunat perempuan or "female circumcision" that has been held annually since 1958 by the Bandung-based Yayasan Assalaam, an Islamic foundation that runs a mosque and several schools. The foundation holds the event in the lunar month of the Prophet Muhammad's birthday, and pays parents 80,000 rupiah (£6) and a bag of food for each daughter they bring to be cut.

It is well established that female genital mutilation (FGM) is not required in Muslim law. It is an ancient cultural practice that existed before Islam, Christianity and Judaism. It is also agreed across large swathes of the world that it is barbaric. At the mass ceremony, I ask the foundation's social welfare secretary, Lukman Hakim, why they do it. His answer not only predates the dawn of religion, it predates human evolution: "It is necessary to control women's sexual urges," says Hakim, a stern, bespectacled man in a fez. "They must be chaste to preserve their beauty."

I have not written about the 2006 mass ceremony until now. I went there with an Indonesian activist organisation that worked within communities to eradicate FGM. Their job was difficult and highly sensitive. Afterwards, in fraught exchanges with the organisation's staff, it emerged that it was impossible for me to write a journalistic account of the event for the western media without compromising their efforts. It would destroy the trust they had forged with local leaders, the activists argued, and jeopardise their access to the people they needed to reach. I shelved my article; to sabotage the people working on the ground to stop the abuse would defeat the purpose of whatever I wrote. Such is the tricky partnership of journalism and activism at times.