The deal is that you must come up with the $150,000 for the child's place. Tough to come up with that kind of scratch when you first arrive there and on an entry level salary. You don't get it back, until your child leaves/finishes school and you can sell it to someone else.
Some grades/level's are fully occupied and you may not be able to find a place for your child. Debenture's are bought to reserve a place for a child often times two years before a child is of school age...
If you have more than one child, you may find a place for one, but an older child may not, and would have to go to the "Hong Kong Side" to attend school.
That was the case with neighbours of ours in DB at the time. Two years of one child going to the DBIS and the other child going to a school by Ferry every morning... Plus they had to pay the debenture for the "reservation" of the place for the child should a place open up...
Tricky, and expensive...
Good luck.