On a little side note, and coming from an English major, your thesis doesn't have to be just ONE sentence. A thesis can appear through the "introduction" or even throughout the paper (although you wouldn't want to do that for this particular paper because it seems to be research). Your teacher is right to say that what you have now is vague--it's a good start but sooo broad that I wouldn't know what the hell you were going to be talking about. What I wonder though, is why she didn't help you start to shape this thesis? Also, what level class is this? College? Upper or lower level (the difference between a 100 and 200 class)? Sometimes teachers at that level see a paper that is structured as I Intro II Body III Body IV Body V conclusion somewhat not mature enough--you know what I'm saying? But that could just be coming from me--English classes run themselves differently than say, a history class.
Anyways, I probably didn't help anything but I just wanted to tell you that you don't have to limit yourself to one sentence! (unless of course that is what your teacher told you
)