First off welcome (that's ONE L in welcome LOL) to Hong Kong from a fellow Chinese Canadian.
Are they asking for your expected salary in the job posting that they list? Most companies in HK just list that out of the fact that almost everyone writes that in the job ad. In all my applications to the jobs that I got hired for, I have never listed it on my CV. The interviewer or HR will request that information after the first interview OR when they arrange for the first interview. I try to ask around or see other postings of similiar jobs on jobsdb and what other companies are offering; it's not that hard, just ask around a few friends and family members and I'm sure someone know someone who's a teacher. And of course as usehername says, you can always put the question back on them. Say that you are new to Hong Kong so it would be unreasonable to expect the same level of compensation from another country. My wife works at a playgroup center and I'd say if they are offering you less than 20-18k just walk for a teacher role (TA maybe less?); apparently there is a big shortage of qualified teachers in HK (not just some fresh grad coming to HK for a few months looking for some cash to party with, which my wife has seen LOTS of...).
Along those lines, if a company or school in your case keep pressing on the expected salary even before they interview you then they are not the employer you want. I had to hire someone in my latest position and when I saw someone listed their expected salary that's out of my budget, I interviewed them regardless to see if that means I need to increase my budget to get a quality staff that can help my business grow. There's no point in hiring someone within budget that in actual fact drags down my business. If your potential employer doesn't get that then you don't want to work for them.
Good luck!
Oh and I know what you mean (LOL I'm going to dip my toe into the heated discussion here); education centers like to put a white face on their ads and to parents. My wife had to fight for her pay grade that is the same as a college drop out from the UK. But give it time because if you've got skills the employers will see that and you will have leverage. My wife now has more students than her colleagues because the children's parents sees actual grades improvement from her structured course (we both have a BA in Psychology and she got a Montesourri (spelling?) teaching certification) rather than some overseas fresh grad who just wings it every lesson with no plan. You will find a lot of westerners who will get offered a job and visa immediately because Chinese parents seems to think that a Caucasian teacher will be a better teacher. There are good Caucasian teachers but one is not indicative of the other. Again, work hard, get results and your employer will listen to your demands down the road.