One thing you could do is to focus on controlled speaking activities at the beginning. I don’t often advise teachers to do this but in your case, I think there are many advantages to this:
the focus stays on the teacher meaning you stay in control,
students might speak more readily if the activity is teacher led,
they might not be ready to participate in debates etc just yet,
they might be more used to teacher-led lessons due to their experiences of the Chinese educational system.
When the students are used to you and used to speaking English you can introduce more communicative activities.
You can use jazz chants by Carolyn Graham. These help students get used to the sounds and rhythm of English and give them an opportunity to practise the language in a safe environment. They are also fun. You could also use songs for the same aim. After listening and working on the language, students could sing along.
You can also start work on pronunciation through chorus and individual drills. The teacher provides the model and the students repeat; or the teacher provides the prompt and the students produce the target structure. For example, teacher: “I went to the cinema yesterday”; students: “where did you go yesterday?” Limit this sort of work as it’s not very interesting if it goes on for too long.
You can have the whole class working on a dialogue together. Vanishing dialogues and spontaneous written dialogues are useful methods for getting students speaking. For a description of how these work, look at my reply to Webmonkey on December 5th.
When students are more used to you, you can divide the class. Have half the class working on speaking activities and the others reading or writing. Deal with the “speaking” section of the class as you would in any communicative activity. Make sure the others have something to do that will keep them quiet. It could be reading or writing as preparation for the speaking activity. If you teach for 40 minutes, each group should get 15 minutes of speaking and 15 minutes doing quiet work. The rest of the time will be used for changing over, giving instructions, etc.
As time goes on, you can appoint students as “language policemen”. The role of these students is to ensure that everybody in their group speaks English at all times. Give them the necessary phrases to do their job; e.g. “please speak English”, “stop speaking Chinese”. You’ll find that teenagers are often more strict than the teacher is.
Lastly, I think Macmillan publish a book for dealing with large classes. I think it’s one of their Handbooks for Teachers series. You can also look at my reply on dealing with mixed ability classes and my reply to Trisha on 12th March about getting students speaking.