I will once again be going though IVHQ for my volunteer placement.  (You can read my review of IVHQ here.)  The following is from their website. 

Volunteer English teaching in China focuses on developing conversational English skills with Chinese students. IVHQ presently works with Chinese public schools; most of which are located in rural areas just outside of Xi'an and serve the farmers' children and poorer people living outside the cities. In China, there is a growing demand for an English education. With the unprecedented economic growth China has experienced in recent years, the importance of English education has also grown. Many schools in China have established an “English corner”, a specially designed spot for teaching/learning English. These schools are very eager to have native English teachers. Unfortunately, only private schools with huge budgets in city areas attract native English teachers. Most of the resource-poor schools remain hopeful for quality English teachers for their classes, with native English speakers at the top of their wish lists.

Xi'an XiGang primary school was founded in the 1980s and used to be the school for Xi’an Iron Company. Now it is one of the Government owned school for the families around this area in suburban Xi'an. The school has around 900 students aged from 7 to 12 years old; they have 6 grades and 24 classes. All the classes have English lessons and the children are all eager to learn English well since it is becoming one of the most important subjects for Chinese students. However, the school has only three English teachers for all 900 students. Compounding this problem is that even the teachers themselves have not really had a chance to speak to a native English speaker before and their spoken English need to be improved drastically. Volunteers are badly needed here not only to help students but also the teachers. NB - This is just one of many teaching English placements IVHQ has in China and is used here as an example.