If your good in the subject and you can teach it well then there shouldn't be any problems. You could always be a tutor! But that all depends on how good you really are if you think your really good chances are you will be able to switch over to subject teaching.

I would imagine you need a degree in the subject you want to teach.  The jobs I've seen advertised all require it.  

You will need further qualifications and what those qualifications need to be will depend on your home country. I'm pretty sure in most countries to teach at high school level requires a primary degree in your subject area: for example, if you want to teach physics you'll need a bachelor of science undergrad degree.

If you love teaching but want a China exit plan, then maybe look at doing a masters in education or something similar by distance, it will help you while you're here and also make you employable when you leave (this is assuming you already have an undergrad degree, if you don't then get one). If I were looking at teaching long term rather than just medium term, that's what I'd do.