Smile. Be encouraging. Be enthusiastic.
Be creative in how you handle things. For example: If you ask a question and no one responds, one way you could respond is to say something like, "I'm not afraid of silence. I can wait all day." But that has been used a lot. A different way to respond might be something like: "Oh goodness, you guys forgot how to talk." Or "Okay, everyone is too tired to talk, let's do this differently."
Then do something which will be easy for them to respond to...like maybe saying: "Okay, quick quiz: best restaurant in Bryan/College Station?" And let them respond. Or any other super-easy question which is sure to get a response out of them. Then say something like, Okay now that I know you guys can talk...(and ask the question again (maybe rephrase it in a different way.))
Another example to get them talking: If you go to Bryan HS, say, "I do." And let them respond. Then: If you go to CSHS, say "I do." And on through the high schools. This gets everyone saying something. And you can have fun with the people who do not seem to be proud of their school. etc. And then ask what schools you missed...and you'll catch the Navasota and Brenham students and other schools.
Anything like that which kind of catches them off-guard seems to help. Generally speaking, they're not as great at talking about Bible things or faith things as I would like them to be...so I try to be creative in how I help them talk about those things.