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.

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.

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

The full story with be available in subsequent episodes of World of Secrets Catching My Teacher

Watch the full investigation on iPlayer from 0001 Monday 21 September. If you are outside the UK, you will be able watch on YouTube

Yusron began investigating Abah a year previously, sparked by a revelation one night by his former classmate Ziadatur Rahmah. They had both been star pupils at Lombok's Abu Barokat school. Now they were at university - Yusron in the capital Jakarta, and Ziadatur in Cairo, Egypt - and had begun reminiscing over the phone about their school days.

One night Ziadatur, known to her friends as Zia, told Yusron that Abah had sexually assaulted her when she was 16.

She had needed to go to the toilet during class and Abah had intercepted her in the school gardens and asked her to accompany him to his house which was on the premises.

"He pulled me down so I was sitting on his lap… Then he started kissing me… He was stroking and touching my body," she later told the BBC World Service, too, explaining that he told her to do the same to him.

Luckily, she said, one of his children began banging on the locked front door. Abah signalled to her to leave and she bolted out the back. But the effect was devastating.

Yusron, when he heard Zia's account, was unsure what to do. Their school had inculcated them with the motto: "Sami'na wa ata'na" ["We hear, and we obey"].