You just have to go with the standard rule of: Watch what they do, don't listen to what they say.
You'll never know her reasons. Some, all, or none of what she says could be true. But it doesn't matter because regardless of the reason, there's nothing you can do is there? If she says it's because of the student organization, would you really leave that organization just to see if it would cause her to date you more? What if she's lying about that? What if it's an ex that she's not over? You can't rationalize anything with her about her ex. What if it's fear of not being ready for the complexities of a relationship yet? Again, you can't rationalize with her on this subject to change her mind either.
A girl with high-enough interest in a guy will not care about an ex, a student organization, or the complexities of a relationship. End of story.
So to h3ll with what she says: Pay attention to what she does. If you like making out with her and fooling around with her and are ok with just doing that and not having full-on s*x then keep doing it. But don't get emotionally invested in her and don't stop seeing other girls. If you don't think you can handle those two things while seeing her then I suggest cutting it off.
From what I see, it sounds like medium interest that isn't growing, and she's biding her time until someone who really knocks her socks off comes around. She could be pining for her ex to come back, or even banging her ex now. Bottom line is don't get emotionally involved in her and keep dating other women because this one has almost no chance of leading to anything from what I see. You are definitely doing the right thing by keeping distance and not being overly eager or responsive with her, so good job on that.