While us men tend to have pretty simple minds, women can be complex creatures - it's part of their charm!
In a situation like this, guy tend to think of the first (and most obvious) thing that comes to mind - that shes not interested in your anymore. While that would make sense given the signs, it may be something else entirely.
There could be any manner of reasons for her being this way:
1) Maybe she is losing interest / has another man
2) Maybe she has something going on in her personal life that she's not comfortable telling you about (e.g family business, health issues, etc).
3) Maybe you did something to upset / anger her, and you just didn't realise it. Women have a tendency to say nothing when you do something to annoy them - many don't like confrontation so they just go quiet, get distant, pull away. The problem may not be her - it may be you.
4) Maybe she is legitimately more busy than you realise. Most people have general day to day things they go through they they don't talk about simply because it doesn't seem that exciting or relevant.
5) Maybe she is stressed about school, and she's trying to avoid distractions and focus 100% on the task at hand
Truth is that people in general are complex, and jumping to conclusions without digging deeper is dangerous business. You might find out she has another issue that isn't related to you at all, that is really upsetting her / bothering her...and if you start making assumption and throwing accusations it might just set her off and push her away.
My advice is to have that conversation, and try your best to be as open and understanding as possible - hear what she has to say. If she keeps avoiding the conversation, then explain to her that the reason you want to talk about it is because the relationship is meaningful to you, and you don't want to see it die. That might be enough to trigger something in her head and help her understand the extent of your concern.
If she seems disinterested and seems to not care, or she avoids the subject, then be willing to walk away. You've done your part - you've been a man, taken action and tried to do what you can to fix it. If she isn't interested in doing that, then no point in wasting your time.
Of course that's purely my opinion (and how I would handle it) thought!