Sorry Smack, The standard example of the contract used by most schools/colleges/universities these days says 70% of the monthly salary in Yuan can be exchanged.
The easy way around any exchange difficulties however, is to take any one of your students into the Bank Of China and stand with them while they exchange your Yuan for any other currency US$, Euro, Pound Sterling, Aus$ etc. Chinese citizens are allowed to change Yuan up to the value of US$ 50,000 per year simply on production of their I.D. card.

The way the Yuan is rising and the US dollar falling in these hard times, it will soon be 1 to 1 with a bit of luck and we'll all be rich on our RMB salaries. (some hope ! )