Good luck, Stevieboy.
Someone once told me that if you are ever going to ask for a raise, then you always have to be ready to leave the job.
What if the powers that be turn down your request–just flat out say no. What are you going to do? If you continue to work there (and it sounds as if you like it there), then will you be seen as someone who can be told what to do, and apparently is willing to work for less than he feels he deserves.
Also, if you want a 20 percent raise, does this mean you are going to start by asking for a 20 percent raise? or will you have to start at say 30 percent and allow them some face when you negotiate?
I don’t think it is going to be as simple as saying I want 20 percent more, and then they just give it to you. It will be bu hao yi si, jing ji bu jing qi, SARs, wait until next semester, we just gave out Chinese New Year’s bonuses (do you get a New Year’s bonus?), and other excuses, but since you are such a good teacher and you have done so much for the school, then the absolute most they can give you is a 5 percent raise. (And then they’ll probably expect you to do even more for the school (yin gai de.))
I wouldn’t ask unless I knew that I could walk out the door–so you can negotiate from a strong position. If they know you’re “soft-hearted” and will stay, they’ll never give you 20 percent.
Best of luck (i hope you prove me wrong), and let us know how it goes.