This late in the year, such a transfer may make graduation impossible. There may also be laws in your state that limit your ability to change schools without parent permission (even at 18). Are you enrolled in a special education program or some other special program outside the mainstream?
Now, if you want to leave home, you probably can. Just walk out the door with whatever they will allow you to take with you. You can stop going to school and take the GED if they will let you, or enroll in an alternative program.