I did a quick interweb thingy search.
ESF Base Rate:
Primary schools (Years 2 to 6) - HK$83,800
Secondary schools (Years 7 to 11) HK$117,600
Secondary schools (Years 12 to 13) HK$123500
Aussie school (as there a lot of those types in CX, apparently they educate the K1Z1s as well):
Primary school HK$129,600
Secondary schools (Years 7 to 11) HK$149,600
Secondary schools (Years 12 to 13) HK$194,200
So 90% of the base rate, and then don't you pay tax on allowances? Say late secondary, as most kids end up there, you are out of pocket at least HK80000 per year. A bit different (ignoring the tax effect) from your 30-40k hkd.
Oh and how is the HKD500,000 debenture being funded? Same way as the 2 months rent deposit and the 1st months rent in advance and the savings for the provisional tax bill? It really is a hard slog financially if you have school kids.