Talk to the school counselor. You are her parent, and not her friend. You have to be on top of it. Yes, transitioning to high school can be difficult, but it is not an excuse to let things slide, especially if she is using lazy as an excuse.
I have to say it, only because I have been working in alternative high school for over 12 years, but do you think there is a possibility of drug use? And if she is not using now, are her new friends using, and she at the very least picking up on their behaviors?
I don't want to scare you or sound accusatory, but drug use is so common these days, it's scary. I worked with a group of recovering addicts a few years ago, and my question to them was always "What can I do so that my own kids don't fall into this lifestyle?" and their answers were always the same--NOTHING. Yes, there are the obvious things that we do for our kids, and you are doing much more than many parents and you are single. But a lot of the kids that I worked with had parents who did "all the right things" and they still ended up falling into the patterns of failure.
Talk to the counselor. Keep the lines of communication open with your daughter. If you can afford tutoring for her, shop around. There are a lot of promises out there, but if she's been a good student, don't get scammed. Also, the school offers help a lot of the time.
Good luck. Don't give up. It's just a phase, but not one that will just go away without adult intervention. (That's my opinion anyway.)