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

Warning: This story contains details of sexual assault

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.

World of Secrets: Catching My Teacher

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.

Once inside, he had told her he loved her, she told Yusron.

"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.

"I felt very dirty, ruined, and I was lost."

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"].

But he also could not stop thinking about it.

So he suggested that he and Zia, together with her school friends Wanda Hamidah and Mahima Eka, gather corroborating evidence to support her testimony.

And slowly, as they spoke to their old classmates, other recollections emerged - the friends sharing their findings with each other on WhatsApp.

The most significant was from a friend of Wanda's. She recalled seeing a younger girl crying in the school bathroom during their final year, late at night. And that Abah had come looking for her, red-faced and panting.

This girl, Sukma Rohana, was still at the school. She told the BBC about her ordeal.

When Sukma was 17, Abah had asked her to join him for new year's prayers, she told us. When she entered Abah's house, he had covered her mouth and told her to lie down, she said, before he stripped below the waist.

"He kept talking, saying he couldn't control himself any more," Sukma said.

"He was feeling me up through my knickers… He tried to rape me."

And then, she said, she had found the strength to kick him hard in the stomach and took the opportunity to flee.

As she ran, she said she had looked back to see his torch seeking her out.

As evidence like this emerged, Yusron would co-ordinate it - following up with calls to the students making the allegations, and keeping painstaking notes.

He typed up a seven-page document outlining the alleged abuse of Sukma, Zia and four other former students.

Wanda organised a meeting with representatives from the Nahdlatul Wathan Diniyah Islamiyah (NWDI) - the powerful organisation that ran their school and hundreds of others, and which was also funding some of the students' studies in Cairo.