'But I will persist as long as I am still breathing, even if it means waiting four decades,' Zhou said in a telephone interview.

The Juyuan school was one of the worst-hit by the quake, with nearly 900 pupils buried in the rubble and at least 300 confirmed dead later. The exact number of dead children at this school or at hundreds of others across the quake zone remains unclear. The government only provided a total death toll a year later, which was widely accused of being too low.

Like other parents, Zhou blamed shoddily built buildings for her son's death and the unusually high number of casualties at schools.

Parents from the Juyuan school, which hit the international headlines after a visit by Premier Wen Jiabao just hours after the quake struck, were among the first to take to the street to protest against injustice in the days following the disaster.

Witnesses said they were inspired by Wen, who pledged to investigate allegations of shoddy school construction and seek justice for parents of the dead students, while standing in front of the school's debris.

But Wen's promises have not been delivered on. And the unresolved controversies over 'tofu' buildings - referring to shoddily built schools - and the death toll among schoolchildren have been deemed taboo topics.

Zhou was also among the first to be subject to frequent harassment and persecution because of her persistent demands for redress for the children who were killed.

She said she had been detained at least six times over the past four years, including once when she was held for two weeks in a black jail in the capital in July 2009, by thugs apparently employed by the Dujiangyan authorities.

'They told me they were paid to teach me a hard lesson by public security authorities in my hometown, as part of the retribution for my insistence on petitioning the central government,' Zhou said.

Zhou was beaten and tortured for hours in late March and then was detained for a week in police custody in Dujiangyan. Two other parents who tried to voice grievances online were also detained for seven days and 16 days respectively.

Zhou's first experience of incarceration came just two months after the quake hit. She was held for two days after she refused to sign an agreement on 'condolence money', which included payments from government departments, as well as help with children's funerals and with parents' employment.

'But in return, parents had to promise not to seek redress for their grievances, especially over the tofu buildings,' she recalled.

Zhou said her wish on today's fourth anniversary of the quake was to commemorate her son at the site of the ruined classrooms, where debris had been cleared and new buildings had gone up.

But like all the schools affected by the quake, the Juyuan school is tightly guarded on sensitive anniversaries. No parents of dead students are allowed to get close.

'I haven't been able to visit the school since the quake. I was even arrested several times for making futile attempts,' Zhou said. 'But I have nothing else to fear. My heart was broken by the loss of my only child.'

She became even more dedicated after her and her husband, also in his late 40s, tried to conceive but failed.

Other parents, campaigners and volunteers who offered to help investigate these contentious disputes have also become targets of a government-led crackdown, along with the mainland's other rights activists and dissidents.

At least three activists - Tang Zuoren, Huang Qi and Liu Xianbin - have been jailed on subversion charges for their investigations into the tofu buildings. Artist Ai Weiwei , who also tried to look into the shoddy school buildings, was arrested in March of last year.