Bonjour Pjkl,
the difference between the monthly gross and net salary is 5% with a maximum of 1000 HKD/month. This is for the retirement scheme. This sum is given back to you when you leave HK. Then you have to pay the income tax (comme l'impot sur les revenus en France). This depends on how much you earn, how many people you have to support in your household. You can go on the following web-site from the French Consulate and download their guide for newcomers in HK "S'installer a HK et Macao" (dans la section publications). You will find very helpful information on HK, including how to calculate your tax.
www.fccihk.com
Re the housing allowance, go for it. Anyway, the more you can ask to your employer, the better. The amount depends of your position in the company.
For the health insurance, if your employer does not pay it, you can check on the internet how much it costs. I've been told by an other French in HK that Goodhealth is OK. And you have the option to go to the French health insurance CFE (caisse des francais a l'etranger qui est rattache a la secu fr:
www.cfe.fr) + mutuelle. You may find cheaper health insurance in HK, but I personaly choose the CFE because I can be paid back when I'm in France and it works like the Securite sociale. I think they have a family package. You will find their price list on their web site.
You may know already: housing and health care are more expensive in HK than in France.
For the school, here is the web site of the French school in HK:
www.lfis.edu.hk
Bon courage!