If you will be working in a Chinese school, you will not be expected to take gifts for the people you will be working with or for, but it is a nice gesture. The cost of the gift is not important for teachers or students (though if you make a gift to your school, quality is important). It also isn't important to try and find something impossible to find in China (especially since you can find almost anything in China). I found the best gifts to be those that had a American aura that can not be found in China. Often, simply having something from America is special enough.
Children are easy to pack for. For my primary school students, I brought stickers, coins, and stamps and the kids loved them. Stickers are especially good if you get the packages with American holiday themes. These sorts of gifts are also good because they don't take up much space in your luggage! I also bought samples of American candy, like candy canes. Be careful about candy because Chinese students have access to things like Snickers and M&Ms. Even older students enjoy stamps and stickers. A high school teacher I knew used pencils or American lollipops to reward his students.
Teachers and co-workers are harder to pack for. Your primary contact will probably be with English speakers, so bringing English language gifts is probably a good bet. Chinese citizens do have access to English classics like Dickens and Shakespeare and classical Western music, but have very little access to current magazines, books, or music. I brought tapes and books to give as gifts. The tapes were taped directly from American radio. The teachers enjoyed listening to current music and talking about the advertising was interesting. Any type of book will be appreciated, but pictures of American life and novels were particularly popular with people I knew. When you pack for living in China, I don't feel it is worth packing a lot of cloths or "necessities" like toothpaste, which are cheap and readily available in China. Even if you are "overweight" by Chinese standards, getting tailor made cloths is Very cheap. Packing books gives you something to read in your free time, and then you can give them away at the end of your trip. You'll have more space in your luggage for your Chinese acquisitions and everyone is happy!