Hi hatsunemiku (presumably not THE Hatsune Miku, but apologies if you are!),
Welcome to the forums. I'm so glad you found us here and were willing to share some of what you've been dealing with recently. Girlfriends, exams, moving, new school, and very serious family drama: a tough combination for anyone, especially a young person like yourself. It's no wonder you're stressed and don't want to deal with any of this! File under: high school crap that sucks.
The most important thing is the physical safety of yourself and your mother, and hopefully both of you have the resources you need to ensure this and resolve the more urgent domestic problems. However, it sounds like now that you're with your Nan you have a tough decision to make.
You sound very mature, as many young people wouldn't be able to clearly see the pros of going back to your old school, the convenience, etc. These are all very important things to consider. You mention that the schools are about an hour away. How would you commute between them? I wonder if you've shared any of these thoughts with your girlfriend, and gotten her take on things.
Would love to hear more.