Still, the debenture market is extremely hot. Kwok, of BA Marketing, only managed to get hold of four "notes", two for the ISF, one for Harrow International School and the other for Victoria Kindergarten.

"We managed to get more after the graduation season when parents transferred the notes. Prices fluctuate with demand and supply ratio changes," he said, adding that parents can get the ISF debenture from him for HK$6.2 million. That is HK$300,000 cheaper than by buying from the school directly, excluding 1 percent commission fee.

Speculation on debentures exists, educational experts have confirmed, but they deny knowing people trading in them.

Usually there is a panel that determines the admission list at private schools, which should level the playing field. But one parent, who declined to be named, said he was trying to put his daughter into an elite kindergarten last year when he was told that more than half of the positions had been booked.

"Many rich mainland parents come to Hong Kong with a great deal of cash," he said. "Spending 10 million yuan on child's education is easy for them. But it is not just about the money, it's about knowing someone that has the power."

Hong Kong has the fastest growth rate of high net worth individuals than anywhere else in the world, according to a report released by Capgemini and RBC Wealth Management in 2014.

Also, personal wealth among mainland Chinese is expected to swell by 80 percent to $40 trillion by 2018. This is compared to just 17 percent growth rates of personal wealth to $54 trillion in the US, the report showed.

Many of the rich families in the mainland are diversifying their assets, and Hong Kong is becoming one of their favorite destinations.

"Hong Kong and the US rank high on the list of destinations for rich people who value their children's education as well as protecting and growing their wealth," findings released by China Ultra High Net Wealth Report, issued by China Minsheng Bank Corp, revealed in April. "Switzerland is the top choice for people who crave confidentiality."

Leading international schools

Independent Schools Foundation Academy

Debenture: HK$6.5 million ($838,500)

Tuition fees academic year 2014-2015:

Primary school grade 1-5: HK$150,400

Secondary school grade 6-10: HK$174,700

Grade 11-12: HK$189,900

This school is particular popular among mainland parents as it provides a bilingual Mandarin and English curricular.