This is from the British Foreign Office advisory on China

An increasing number of British nationals are becoming attracted to opportunities to teach English in China. Most of those who do so have an extremely positive and enjoyable experience. However, some have experienced difficulties. The most common problems encountered arise from being faced with living or working conditions that do not meet expectations and complications over obtaining the correct visas and residence permits. There have also been complaints of breach of contract, confiscation of passports and of payment being withheld.

If you wish to take up a teaching appointment in China you are advised to contact the nearest Chinese Embassy for information on obtaining the appropriate documentation. It is illegal to work on a tourist or business visa. Thorough research on the educational establishment and the area in which you intend to work will pay dividends. Further information can be found in the consular area of the website of the British Embassy Beijing.

More information here.