Maybe like she is bothering someone who doesn't want to be bothered. You might not be the only person who finds her behavior annoying. You may be the one who doesn't send her away.
Especially at night school, most people are there to get the classes done and not much more.
I do like Roadie's ideas. But, I would also say, just be yourself, don't respond to her "is it something I did" behavior. There would be nothing wrong with having something else to do when she comes along (reading for class, texting a friend....). You are under no obligation to be friendly, or more than just civil. I think, if I am "hearing" you right, the trouble may come when you try too hard to be nice and then the person makes it look as if you had been "welcoming" and then "turned" on them. And, do Not tell her anything personal.