What should I do in this situation? (extremely long details inside)

Sorry for being too lengthy but I just try to be as detailed as possible. I think it will help you give the best advice possible. Please be patient. I urgently need your help.

First of all let me review the scenario: I have a friend who used to be my classmate at high school. Now she is my university classmate. She’s a very bad-tempered person and when she gets angry, you can possibly guess what happens. She is also kind of childish, sometimes she gets angry with things I can’t understand. We became friends because we sat next to each other at high school. We chatted with each other and occasionally helped each other with our learning. I befriended with her because she was the nearest one to me, though I don’t really think she and I are a good match because of her “weird” character.
At high school she was sociable and enthusiastic. Once she lectured me about the importance of being sociable and responsible (I was very shy back then). But at university everything seemed to turn upside-down: I become super sociable and quickly formed a group of friends with some students, whereas she becomes rather cold and distant. She skips class frequently and tries to avoid group work. At school she chooses to sit in corners where the teachers least notice and do things completely unrelated to the lessons like learning something else or acting bored. No one in my class apart from me seems to like her and there is a rumor among my friend-group that she is planning to move to somewhere far away.

OK, let’s get to the main point.

About a month ago, my weird friend asked me to take her home. That was when the rumor was confirmed: she told me that her family planned to let her study abroad, so she was currently studying “some necessary things” (I guess she intend to get some kind scholarship to save money). Therefore, she was so busy that she could not attend class regularly and do her homework. She then made a deal with me: I did the homework she told me to and handed them down to the teachers in a way that tricked them into thinking she did it, not I. In return, she promised to “pay me back”. I accepted the deal; firstly because I felt rather sorry for her having to prepare for the trip and dealing with schoolwork at the same time; and secondly because I thought she would only ask for help when she was really short of time. She asked me to kept our deal a secret since she was not really sure if her plan would succeed, which I did.

But as time went by I quickly realized I was (possibly) wrong. She asked me to do every homework coming to us, some on which, I know for sure considering her ability, were easy enough for her to do within 15 minutes. What’s more, she even asked me to do some tasks which didn’t seem to be included in our deal at all. For example, one day her English group held a discussion about their upcoming presentation in a certain place (the presentation was an assignment), she told me to attend the meeting on behalf of her and tell to the group she was “too busy to go”. Another time our English teacher asked us to do a research into a certain topic then write a report at the class. My friend told me to write the report beforehand and gave it to her at her home so that she would rewrite it and I would hand down to the teacher, tricking him into thinking my friend had attended the class and had written the report properly (she didn’t go to school at all that day)... I once asked her why I had to attend her group’s meetings and she was like: “I’m busy! You already know that, don’t you?”

I soon felt really annoyed about having to do everything for her, especially when I had a lot of work to do and her extra tasks only made the matter worse. But I just tried to persuade myself that she was just too busy and I was just doing a favor for her (I think I’m kind and ultra-tolerant). It was until three recent events happened that I’m certain that my tolerance is limited and I have to do something.

Firstly, a week ago she put her “fee” inside my schoolbag and told me to see it at home. There was a bag of snack and a package of candies. The snack was of the cheap kind and it was available everywhere. As to the package of candies, I noticed some dead ants inside it. My speculation was that she had already eaten the candies before giving it to me. I was frustrated, but I didn’t tell her in case she would thought I was too demanding.

Secondly, several days ago our English teacher gave each of us a unique, heavily personal-opinion-based writing topic as a mid-term test. She gave me her topic and ask me to write it for her. This is a mid-term test and it is personal-opinion-based, how can I do it for her?

Thirdly, just 2 days ago our computer science teacher asked us to translate some English computer-related documents. The translation was totally optional and anyone who participated would receive bonus marks. She agreed to translate, but then sent me the pages she had to translate and ask me to do all the translation for her. Since it was just an optional task, I was really irritated (and that’s ultimately what drove me to post the question here).

At this point I could have canceled our deal. But I just can’t. As I had stated before, she’s a very bad-tempered person. I was her friend at high school, so I knew how her anger would be. I’ve seen how she vented her anger on me and the other. I don’t know why I just can’t pluck up the courage to tell her straight that I don’t want to continue with our deal anymore and stop everything. But I don’t want the situation to continue. I just can’t stand seeing her get all the fruit while I have to work hard for both my tree and her tree. She may be busy with her work, but is she really that busy to have me do everything? Not to mention she seems to be asking too much from me and my tolerance is running out. Now I’m in a real mess. Could somebody here please give me some advice about this situation?