It's the features of each investment that vary to fit the needs of each school. It therefore becomes a bit of a headache for parents looking to find not only the type of education but also the payment system that fits their needs.

So, how does a family with a child to place in an international school go about it? Since it's difficult to classify debentures by the features provided, the only way is to check out each school's system and see what options are available.

Some international schools allow their debentures to be bought and sold on a 'secondary market', and those that do often vet the sales and keep half of the profits. For parents who go through the process, the payoff can be quite handsome.

'There are some [parents] I know of that have bought them at the HK$90,000 level and are selling them at HK$400,000,' said Alex Gibbs, communications and development manager at Canadian International School. Mr Gibbs, who oversees the negotiation of corporate debentures, said the school stopped issuing debentures after it had issued 900.

Holders of those individual and corporate debentures are selling them on the secondary market for between HK$380,000 and HK$420,000.

'It's an investment by the parents into the school, but it's [also] a financial investment,' Mr Gibbs said. Debenture transfers are subject to the school's approval, but once approved, the school and the selling family split the profits.

School oversight meant parents did not pay too much for a debenture, said Mr Gibbs, but it also means they won't get it for a steal. 'It's sort of a controlled market by the school,' he said.

Sometimes a school matches families on its waiting list with willing sellers whose children have already graduated, and occasionally a family 'retires' a debenture by allowing Canadian International School to reissue it at market value and keep the difference in price rather than splitting the profits. Mr Gibbs characterised it as essentially a thank you to the school for a job well done with the child or children.

There are also 'membership sellers', such as China Dragon Membership Services, which charges a 1 per cent sales commission. The company normally oversees two or three debenture transactions per year, sales manager Hilary Don said, and they are negotiated on a first-come, first-served basis.

'If both parties agree to the price, then I contact the school,' she said.

However, schools contacted by the Post denied any dealing with this company.

One family advertising a debenture for sale simply posted it on the internet.

'Jump the queue ... DBIS premium debenture for sale!' said an ad for a Discovery Bay International School debenture on the AsiaXpat.com website. It was an informal auction, with the minimum asking price HK$150,000.

Debentures at some schools come with an admissions priority for the child, subject to satisfying the school's qualifications. Hong Kong International School charges HK$500,000, and German Swiss International School HK$250,000.

Parents who find the debenture-trading process too complicated have other options. DBIS operates on a system of non-refundable, non-transferable school development levies, according to the school's website. Families may purchase a one-time, HK$300,000 premium development levy that will give parents 'fast track' entry into DBIS. Those needing a less expensive option may purchase a single HK$30,000 levy, payable in two instalments after the child obtains admission at the school.

Parents at Australian International School may pay HK$400,000 for a debenture with an admissions priority, but if they want to take a chance that their child will be admitted without priority, the debenture is HK$60,000 after acceptance.

At Kellett School, the debenture is HK$300,000 - labelled a corporate debenture but also available to individuals - or, if the family waits and the child is accepted, the debenture is HK$60,000.

Parents who would rather forgo the debenture purchase process altogether can pay an annual levy at some schools, often a cheaper option for students who enrol for only a few years, such as high school.

At ISF Academy, an annual, non-refundable capital levy costs HK$12,000. Parents opting for a one-off payment at ISFA may buy a $100,000 non-transferable general capital levy or a HK$200,000 'capital note' for an admissions priority and the right to sell it later.

In March, Yew Chung International School of Hong Kong began collecting debentures for the first time, priced at HK$200,000, to help pay the nearly HK$1 billion cost of building a new campus, according to a school spokeswoman.

In contrast, Japanese International School discontinued its debenture system, which it started in 1997 as a quick way to cover start-up costs, and now operates on a pro-rated levies scheme, according to principal James Mistruzzi.

'After a few years for us, that huge amount of money coming in as a one-off payment was not something that we needed to continue,' he said.

Most international schools, however, have kept their debenture options, and as long as the prices of debentures keep rising, they are set to remain both an investment and a headache for parents whether they're on the buying or selling end of the spectrum.