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.

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.

The meeting went well, the friends told us - the NWDI representatives had said they would help them take the case to the police.

But three weeks later, when Yusron received that ominous text from his sister - calling back to find Abah on the end of the phone in Yusron's family home - it became clear the NWDI had passed on Yusron's written documents to him.

"If this is leaked to the media… I'll be destroyed," Abah can be heard saying on Yusron's recording of him.

He pleads with Yusron to lie for him, giving him a series of cover stories he can use to retract his evidence document.

He then called Zia and exerted similar pressure. Like Yusron, she recorded the call. In it, Abah admits he abused her but pleads with her to stay silent.

Sukma, for her part, told us she was called to Abah's house to discover two police officers and an NWDI lawyer waiting for her. They forced her to sign a document saying all her allegations were lies, she said. Her account is backed up by a secret recording she later made as she challenged Abah's wife, Hurmiati.

Hurmiati can be heard telling her to respect their "agreement" that nothing happened, adding: "Do you want this to haunt you for the rest of your life? Do you want it to damage the chances of you getting married?"

This pressure even extended to Abah asking Yusron's parents to fly to Jakarta to look for him at university. They didn't find him - he was hiding in a hotel - but the ordeal was too much for Yusron's mother. She told her son the stress was killing her.

So Yusron reluctantly stopped his investigation.

Meanwhile he and Zia were slowly getting closer. The friendship had become a long-distance relationship, and they were regularly speaking on the phone.

"And I could see she was really struggling… She'd say things like: 'Why do I keep thinking about what [Abah] did?'."

Yusron was torn between helping the girl he was falling in love with, and the promise he had made to his mother.

And then, eight months later, Yusron heard about a group raising awareness about sexual abuse in Islamic schools. It had been set up by a young woman called Ayu Masruroh, who had helped her friend get justice after she had been sexually assaulted at another school, by a teacher called Moch Subchi Azal Tsani.

Ayu was now actively posting on social media about the need to speak out.

It inspired Zia to post a call-out on Instagram, asking any Indonesian student who had experienced abuse to get in touch.

And she was to discover that, despite Abah's remonstrations, and promises to change, he had apparently offended again. Someone who went to Zia's school got in touch to say she too had been sexually exploited by Abah. The alleged abuse had taken place almost a year after his pledges to Zia and Yusron.

"It made me realise [Abah] hadn't really changed at all… all the things he said about stopping… It was all just lies," said Zia.

Yusron's parents finally agreed he should help Zia and Sukma to continue their quest for justice.

But this time, the friends decided to do things differently. They contacted Ayu, and she was clear: get a lawyer. She introduced them to someone experienced in representing sexual abuse survivors. He advised that Sukma's case was the strongest to ensure a prosecution.

We contacted Abah and Hurmiati about the allegations against them - they did not respond.

NWDI said it was still investigating the allegations internally and denied ever trying to cover up sexual abuse claims.

It said it had "a duty to safeguard the organisation from being tarnished by the alleged actions of individuals".

Sukma filed her claim against Abah to the police in February last year. She is still waiting for them to take action and Abah is still in post.

"I don't want what happened to me, to happen to any other students. I want justice," she told the BBC.

The investigation has, however, brought Zia and Yusron ever closer. Last February, after three years of messaging and speaking, Yusron decided to move to Cairo to be with her.

"I was exhausted. Jakarta had been intense, because I was dealing with the police case the whole time. So when I saw Zia… it was like medicine," he told the BBC.

Zia said she was so happy she "started talking non-stop.

"That's when I knew he was serious. He had come all the way to Egypt for me. I trust him. That's why I'm with him."

It is a trust built, in part, on their shared pursuit of justice. Last month the couple got married.

Their lawyer, Joko Jumadi, says he has been struck by the students' courage and precision.

The recordings they secretly collected, he told us, capture the pressure to stay silent in real time and are exceptionally valuable. "Several victims are mentioned - acknowledged by the individual himself. These are key pieces of evidence," he said.

Joko and his team have handled 20 sexual abuse cases involving Islamic boarding schools in his local West Nusa Tenggara province over the past three years, and he believes this is "just the tip of the iceberg".