I will answer this question in a total different way (aka frame challenge) as I'm someone who keeps rescheduling as well.
As Wrokar answer says, she did a friendly attempt of getting coffee but she is actually not interested in getting a coffee with you.
She is, either:
- Not interested in you (I would not consider this as a deal-breaker and I'll explain why in a second);
- Busy at all (unlikely);
- Not into having a 10 minute small-talk (this case is very common for me, as I'm introverted/extroverted).
But wait, not everything is lost!
You also added her on Facebook and she send you a message to get a coffee and having in mind how the first impressions were for her, she even might be interested in knowing you better! (Otherwise, she would probably not message you at all.)
Instead of trying to schedule something with her, try to know each other better before anything (break the ice). You also said you are classmates so I'm sure you will see her sometimes at your uni, so, I would definitely try to chat during breaks or you can also keep contact on the social media.
It's really important to not exaggerate on these interactions as it can be annoying for her.
This helped me in multiple situations because people will be more "available" for you and you now can schedule things that you're both interested into.
Small-talk is something I really don't appreciate because it makes conversations boring and uninteresting and, as I am introverted myself, I would really avoid these interactions and keep rescheduling or saying:
I'm very busy now, can we talk later?
Saying no is too direct and might hurt your feelings so this line is a light way to say she doesn't want to talk to you.
Knowing her personality and keep things unscheduled (e.g having a walk after class) will give her freedom to choose what she wants and not be pressured by a date/time to meet up.
I even sometimes skipped meetups just because I was enjoying some TV series at home and didn't want to leave home at all.
This really helps for my friendships because having a scheduled time is stressful while unpredictable things makes me excited about it.
If she is still "busy"/doesn't want to meet with you, she is uninterested in you and there is not much you can do.
I've been on both sides (being uninterested/being busy) and it depends what the other person thinks of you at the moment and
vice versa. I've had situations on long friendships that I just felt uninterested on the other person so it doesn't matter if it's a new friendship or not.