If they are unaccustomed to talking and thinking, perhaps start with some guided conversations.
Perhaps assign them to write up some dialogs with some specific criteria (what tense they will need to use, vocabulary to include, what task or situation they need to resolve).
Don’t just ask them to give you their opinion on international politics. They don’t have one. In fact, few of them probably have an opinion on very much beyond school life and their own family situations. Keep that in mind when creating activities. Asking them to generate conversation in English on a subject which they would have difficulty having a descent conversation in Chinese is unrealistic.
If you want them to do a debate or speak on an issue, give them relevant material to study beforehand.
Do a lot of small group work where students talk to each other. (This requires some serious consideration of how to get the kids to talk rather than sit there silently, but it is a good use of time.)