What country is your student from? I have heard of some students from South American families who come from a relatively wealthy background and who are used to having housekeepers pick up after them. Doesn't mean she should be treating you like the maid, but it would explain her tendency to just leave stuff laying around.
I would recommend that you just sit down with her and talk over your issues. Do a "sandwich" kind of discussion - start with something good about her, then talk about the issues you need to address, then end with good stuff. Also be willing to listen to issues she may be feeling about living with your family.
Hey __[name]___, we need to sit down and talk about a few things. First, I want to tell you that we are really enjoying having you as part of our family this year. You are cheerful and friendly and willing to talk to us. But we need to address some things we are having some trouble dealing with. You may not realize, but our expectation is that you will clean up after yourself when you make a mess or put things back the way they were when you use them. For example, I have had to talk to you about leaving your things on the floor in the bathroom or leaving apple cores and gum wrappers around the house. We don't have a maid service and when you do these things it feels like you are treating me like your maid. Our family expectation is that everyone cleans up after themselves and we really expect you to do the same. It is uncomfortable for me when I have to repeatedly talk to you about the same kind of behavior. We know that every family is different and things may be handled differently when you are at home with your family, but here in our family, the person who makes a mess is the one who cleans it up. We're not asking you to behave any differently than we expect our daughter to behave. Also, I'm happy to make sure that you have what you want to eat for breakfast and lunch, but when I go grocery shopping, I take into account what everyone has asked for and what I will be cooking for the week. If you want to eat anything that you haven't requested or I haven't said is for general use - meaning anyone can have it - please ask before you take it. For example, I bought yogurt for my breakfast and when I asked you if you wanted me to buy some for you, you said you did not. Then you ate my yogurt and it wasn't there when I wanted it. Again, I'm happy to make sure there is enough yogurt in the house for both of us, but you need to let me know that you will want some or I will only buy enough for me. [Add any other things you really need addressed to make the rest of your year comfortable.] ____[name]_______, I sure don't want you to think we don't like you, because that is far from the case - we're really glad you're here and are enjoying learning about your culture and sharing our culture with you. And I'm sure not everything is perfect for you, either. Are there any issues you would like to talk about? We would be happy to discuss them and see if we can work together to resolve any difficulties you are having.
It can be really awkward to have these kinds of discussions. We had a perfect year with our first exchange student - she was polite, funny, talkative and really bright. She was a perfectionist who never had to be goaded into doing her schoolwork (and, in fact, she stressed over any grade lower than an A - I only wish my own kids had taken lessons from her!). Boy, was our second year a shock! Our student was a sweet gal, but hid out in her room with the door closed most of the time. I talked to her three different times about it over the school year. Explained that when she didn't come out and participate with the family, it made me feel like a hotel/restaurant/cab service (because she came out to eat or if she wanted to be taken somewhere). After each "talking to" she'd start out staying out of her room but would eventually ease back into being in her room more often than not, until I had to talk to her again. Couple that with her habit of just telling me she needed to be taken somewhere (I expect to be asked, not told) and usually about 5 minutes before she needed to leave, and I was really ready for her to go home by the time she left. But we're actually still in contact with her to this day - not frequent, but occasional. Our third, fifth, seventh and eighth students were much more like our first - just a pleasure to have around and it only took mentioning something once for it to be taken care of. Number four, however, was a different story. He didn't like to shower on a daily basis and, as a teenage boy, REALLY needed to. It was horribly uncomfortable, but I had to talk to him about it twice. The first time, he must have thought I was joking. But the second, I told him how uncomfortable it was for me to talk to him about it, but that here in America, people are very sensitive to odors and he needed to shower every single day. That did it. And I've already mentioned our problems with exchange student number six - we really didn't know there were problems until she was moving out of our house.
Of course, being a teenager, you might still have to do such things as ask her to look around the car and make sure she's taking anything out that she brought in with her. No kid is perfect! If you have to talk to her more than once, I would just reiterate that what you want is for her to function like a member of your family, and you're just letting her know what your family expectations are.