A debenture is basically a refundable (or saleable) deposit you pay to the school before they will accept your kid.
For DBIS, the debenture is HK$150,000 (refundable) but you can pay a non-refundable debenture.
It's all covered on
www.dbis.edu.hk Also look at
www.esf.edu.hk , specifically
http://www.esf.edu.hk/news/for-parents-fees.pdf
It's all there, you just have to look (try to work out how to use google!).