I know this doesn't have to do with the question at all but I wouldn't agree with that. I've only ever heard of one girl leaving home during the second year of sixth form, and that was because her parents were very rich and bought her a flat for her eighteenth birthday. Also College and Sixthform are quite different. Sixthforms are generally connected to schools, and will be pretty much completely 16 - 18 year olds and focus on A Levels. Colleges offer a much wider range of subjects, often more vocational, and be a wider range of ages.
However just a quick note muggle students go to college at 16 until 18, it can also be called a sixth form and mainly for the first year its living with parents and second most people tend to live by themselves. Just incase you wanted to include any muggle parts to your story!