But in recent years, a steady stream of top government schools making the switch - St Paul's Co-educational College, and Diocesan Boys' and Diocesan Girls' schools among them - has roused growing concern that the trend will widen inequality in educational opportunities.

School fees are certainly higher among elite DSS schools: DBS, for example, now charges HK$40,000 for its primary section and HK$38,000 for its secondary section each year (HK$62,000 for its International Baccalaureate stream). St Stephen's Girls is proposing annual fees of HK$35,000 for its secondary students and HK$30,000 for its primary intake.

DSS schools are obliged to set aside at least 10 per cent of fees received for scholarships and fee remission. But as a critical 2010 Auditor's Report found, several schools failed to do so; one college instead invested HK$71 million in equities and other instruments. DSS schools have since improved transparency and placed details of relief assistance on their websites.

But that has done little reassure people like Joy Liu Shuk-wah, a member of an alumni group opposed to the switch. Liu argues that the move would impede the school's mission to provide education to students regardless of family background and financial status.

"It provides more choice for rich families but not for the poor," she says. The switch would largely limit the pool of students to those from more affluent backgrounds.

Some school-sponsoring organisations such as the Society of Jesus, which runs Wah Yan College, Kowloon, are determined to keep their school public. "We need to cater to students from all socio-economic backgrounds, including the grass roots," says Wah Yan principal John Kang Tan.

"The fee remission strategies may not cover all needy students. There is also the psychological barrier among parents who may not want to apply after hearing the title DSS," he says.

Professor Chou Kee-lee of the Hong Kong Institute of Education, concedes students on assistance may feel stigmatised. But while he found a widening gap in educational attainment among youths from rich and poor families in a recent study, Chou says DSS schools give better-off students a choice: "You cannot deny schools the right to switch."

For the elite Heep Yunn School, joining DSS last year gave it the financial means to sustain activities that it takes pride in, from sports and debate to visual arts, after years of struggle.

It also allocated 30 per cent of its fee income for scholarships and remissions, keeping the same ratio of assisted students. "Our school makes no distinction between students of different socio-economic backgrounds," says principal Dave Lee Chun-hung. "We enable all our students to take part in personal growth training or exchange programmes."

St Stephen's Girls says the switch will enable it to strengthen its curriculum and staff. "We can also use the additional resources to augment smaller teaching units. For example, we now hire nine extra teachers with our own resources. This is unsustainable financially," its council said in an e-mail reply.

The council argues that a major challenge for many Hong Kong schools is they lack the autonomy to devise programmes that can adequately prepare students for the 21st century. "The skills that children need for their future change very rapidly. Therefore, schools need to adapt their programmes in a more timely manner. Schools also need to design more personalised teaching approaches ... which best suit the needs of students."

To maintain a diverse student body, St Stephen's Girls has pledged to set aside 20 per cent of its fee income for remission and scholarships, and adopt a "blind" admissions policy.

"We are determined to ensure that no girls will lose the chance to join us because of the fee," its council says. Its fees will also remain unchanged for the first five years of the switch.

St Stephen's Girls is also keen to gain control over admissions. Under the DSS, it can opt out of the central allocation system for school places, which has made for unpredictable numbers of pupils advancing from its kindergarten and primary sections.

In the wake of an outcry over fairness of the DSS, Secretary for Education Eddie Ng Hak-kim has defended the system. He has said that more than 30 per cent of DSS schools charge annual fees of below HK$5,000; five junior secondary schools are even free.

Education Bureau figures show 26 schools (42.6 per cent) of the 61 secondary schools in the DSS system charges fees of HK$20,000 or more each year, with 15 setting fees at HK$30,000 or above.

But Professor Tsang Wing-kwong, an education researcher at Chinese University, says it is misleading to look at overall figures. Speaking on RTHK's , he says focus should be on the 12 elite traditional subsidised schools that joined DSS since 2002.

Tsang says DSS schools can be divided into four types: leftist schools like Pui Kiu Middle School; private schools like Delia Memorial School that joined DSS soon after its launch, and which charge relatively cheaper fees; schools established after 2000 like Po Leung Kuk Ngan Po Ling College; and traditional subsidised schools that made the switch more recently.

It is the fourth type that is exacerbating inequity, he says. To attract more schools to DSS, the government relaxed funding requirements in 2001. Previously, tuition in DSS schools could not exceed the subsidy amount by two-thirds.

The new policy permits tuition to be set at a maximum of two and one-third of the subsidy. With annual subsidy for a DSS student averaging about HK$40,000, that means a school can raise tuition to HK$100,000.

The legislator for the education sector, Yip Kin-yuen, says another factor for elite subsidised schools making the switch is to exercise full control over intake.

"Students were divided into five bands before. This has shrunk to three, which means students' abilities can vary greatly. By going DSS, those schools can decide solely on students' performance in interviews. A grass roots student without access to many extracurricular activities will certainly lose out to kids from well-off families who have joined lots of programmes."

The Education Bureau concedes there is some inherent bias. The reason many schools fail to use funds set aside to support needy students may be "that relatively more students coming from affluent families perform better in interviews, making them more likely to be accepted by DSS schools", officials told legislators.

Other figures also point to how the DSS system widens inequality in education. About one third of DSS secondary schools set aside part of their intake to include students placed under the central allocation, but none comprise elite schools.

While DSS schools account for 12.3 per cent of total places, Yip says a closer look at key districts where more elite schools made the switch show how access to top schools is squeezed for poorer students.

He cites Central and Western District, where there are 564 DSS places at the primary level, accounting for 4.4 per cent of the total. Coupled with private and international schools, the non-public sector makes up 41 per cent of primary places in the district.

Similarly, 2,896 DSS secondary school places in the district account for 22 per cent of the total. But together, the non-public sector makes up 39 per cent of secondary school places in Central and Western.

This is much higher than the overall average for the non-public education sector, which is 18 per cent for secondary and 21 per cent for primary places.

What this means is a wealthier parent living elsewhere can send his child to a DSS school in Central and Western, but this additional option is at the expense of grass roots families in the district.

"The trend is that the paid education sector in Hong Kong gets bigger and bigger while the free public sector keeps shrinking," Yip says. To improve equality, he suggests the Education Bureau should consider requiring DSS schools to set aside a number of places for central allocation.

"If DSS were wholly private, the public wouldn't care about how they were run, but they get lots of taxpayers' money every year," Yip says. (HK$2.9 billion was allocated for the DSS sector in the 2011/12 budget.)

"The government should adjust the funding model. DSS schools get autonomy over student intake and more money to build better campuses.

"Instead of creating an unfair advantage for the DSS sector, the government should boost quality for all."