When reading this I see my former classmates flashing before my eyes... My guess to improve this situation and get them to talk about more important topics is the why-question. For example, throw into the group: why do you find talking about politics or society not interesting. I can imagine the reasons: boring, not interesting,... But if you get just one or two person(s) to eventually open up to the subjects, chance is the group will start talking with you about it on a more professional level.
Teachers like you can mean a huge difference in somebody's life, sometimes a teacher like you is the only one that can open their eyes to reality for just a moment. Some take that moment with them in their lives and develop it further...
You are not to be judged for working in a public school system (previous poster...), we need people like you inside that system, it can save people from an apathetic life. Just as a teacher was able to diversify my interests in life... From nihilistic to interested and critical.