You feel abandoned and isolated, I can’t imagine how difficult that must be. While it seems like forever, it will be over when you turn 18 and go to college. Do not rebel until you can win. No matter what you feel inside, smile and play the part. As frustrating as it is, the best thing you can do is to comply with your guardian. Do not, for example, correct her when she tells you she is in control until 25. There is nothing to be gained. Just smile and nod a lot until you finish HS and leave for college.
Stay in school, work hard in school, go to class every day, do your homework every day, go in early to see your teachers, ask questions about assignments often, and get to know your school counselor. Be the best student you can be. Get the best grades you can. Especially in English, and math.
Assume that your guardian is well intended, even if she is misguided. In the summer, read as many books as possible. Preferably a blend of fiction and non-fiction, if you can.
Try to get a PSAT study guide if you can, and practice a little every day. If there are clubs available for English, math, or science, try to join any that interest you. By being a strong student, you will create more options for yourself when you turn 18.
Remember that if your school is not the best, the students around you are not the ones you will be competing with at a top college. Try to learn all you can, not just enough to get an A. Remember that success in school is all about effort. The hardest working students almost always beat out the smartest in the long run. Be a hard working student.
By being a strong student and staying agreeable, she may begin to loosen the controls on you somewhat.
By the way, If your guardian likes President Obama, he attended Columbia and Harvard. Perhaps that will make her hate them a little less.
Good luck young lady. Believe in yourself. You can do this. When things go badly, do not get discouraged. Shake it off and try again. You can do this.