I had the same issue last year, although I feel I did not address it properly. My solution was to stay quiet, and eventually, as the problem continued, I got mad and cut her out, and our friendship went on a rocky road. It's recovering now, but that was a BIG mistake that I made. Here is what I wish I had done, and I recommend you do as well:

1. Talk to her, explain your feelings and ask if she has been copying off of you. Don't get mad, just ask her kindly, genuinely why. Maybe there was a reason that she felt it was needed to do.

2. If she gives a reason that asks for your sympathy, as in mentioning something like, "I am trying to manage babysitting my siblings, help in the house, do homework, clubs, and keep my friends happy. I don't understand the class and I am failing, and I don't want my parents to be upset with me". Or something along the lines, then show sympathy. Give her advice in studying, suggest working together after school studying, get a tutor, talk to the teachers or parents, etc. Just show sympathy, that is the most important part here.

3. If she hints toward procrastination, you have two suggested choices:

a. Ask her if she is kinda confused in class, or really busy and stressed. Try to read her emotions and do kinda what I showed in 2. OR...

b. Show sympathy still, along with the lines of "we all feel that way at times, I can help you through it" and kind of lean toward helping her. Even feel free to combine a and b together for this.

4. If she doesn't want to talk about it... well, then it gets difficult. Try to ease into sympathy, and be willing to help. She doesn't have to answer - but try to read her emotions and work off of that. Just be gentle, you never know what's on her mind and the reasons why. Again, sympathy, and if you think she is scared of admitting her faults, maybe bring up a story where you did something similar, or say that even if she did do it, you would understand. Just be sympathetic.

5. If she refuses her wrongdoings straight out, with no hint of lies, it is really time to think. Consider her past behavior - when did it start, was there a big event, and how has she been acting recently? Talk to her nicely, asking what has brought about her new style of learning. This is the part you really have to be careful about, wording this gently. If all else fails, even with trying to work in the other 4 solutions I said, then the last thing to do is bring it to the teacher, but not as "tattling" but genuine concern. Ask how to help your friend, and how to do it nicely.

I am sorry this is so long, but I really hope it helps! Like I said, I wish I had done this last year to save my friendship, instead of taking the path of being cold and distant, in a sense, even a jerk. Good luck, and I wish you the best! 

Ooo. Tough one. I've experienced this before. When I confronted my friend, things grew a bit uncomfortable between us... Aaaand I just read Ashlee's post. Welp, whatever floats your boat. Maybe instead of letting her see the answers, help her find where they are.