Our head teacher was an abuser. We joined forces to get justice - now we're married

The voice on the end of the phone made Yusron Azzahidi's stomach lurch. He had rung his sister's number after she had texted, urgently asking him to call. But this was not her on the line - it was Yusron's former head teacher. He was in Yusron's family home.

"I knew. I knew exactly what he wanted to talk about," Yusron says.

Ahmad Imanuddin Sumar, known to his students as "Abah" ["father"], had a reputation to uphold. He was the widely respected leader of an Islamic boarding school on the Indonesian island of Lombok. And Abah had discovered that Yusron and his old school friends had been collating testimony from several students who say he sexually assaulted them.

Sexual abuse in Islamic boarding schools has been described as a national emergency by Indonesia's National Commission on Violence Against Women. But many cases never reach the courts because the victims are persuaded to withdraw their allegations, or are pressured into staying quiet, legal experts say.

And now Abah was on the phone to Yusron, making exactly that kind of demand.

"Cover the shame of your teacher. Everyone makes mistakes, my child," he sobbed.

What he did not realise was that Yusron was recording his every word.

When a dark secret emerges at the heart of their school in Indonesia, former students turn detective to seek the truth.