This is a problem. Well if they don't feel like talking, they might blow you off.
I learned that people, especially girls, want to talk to other people, they are usually just really nervous. Or tired, or some other thing.
Even I don't talk to everyone, because sometimes, they just don't feel like it. I will usually either talk to someone else, or do something else.
But usually, they are just nervous, and either not very open, or they don't like you, or something, and you can't really help it. Try to determine if it is worth talking to them, or if there is someone else that you would rather talk to.
Sometimes, I just don't talk at all, and just look around the room, observing different people, that might be open to conversation, or that might be looking at me, then I smile, and open up to them. But if it is really lame, and the teacher doesn't notice, maybe you can get some shut eye?