My oldest has just changed schools to a 6th form college.
This time last year she was adamant she was staying on, and I only persuaded her to look round and apply as a back up. She only started preferring the 6th form around Easter and wasn't totally sure until she'd got her results, but, despite hating decisions, she was certain by then.
So she may change her mind.
I wouldn't go at it with "I can't help you because I'm dealing with the younger ones". That sounds firstly as though you don't care about her in comparison (yes, I know exactly where you're coming at it from, but it's very direct) and secondly it does sound like you want to object all the way, which, as we all know with a stroppy teen, is exactly the way to make them dig their heels in. 
What I'd do is go and look round the other schools that are possible. When you've been round then talk about it as an adult conversation. Not "I prefer this" but "What exactly is good and bad about each?" In that bring up how can you get there? It may be that there's a perfectly good bus/train. So you can say "you realise that will add 40 minutes onto your school day" and that can go down as a disadvantage.
Then make a choice but, if possible, if she chooses elsewhere, make sure her old school is still open to her.
I was the opposite to your dd. I wanted to stay and I ended up moving. I still don't know if it was worth it. I got to where I wanted to get to, and I'm not sure I would have in the old school. But my friends stayed on, and I missed out on things I had looked forward to doing when in the 6th form.
But if I hadn't got to where I wanted to get to, it would definitely have been a bad move, and I would have been throwing it back at my parents I'm sure.
Also the reasons why my parents thought it would be better to move mostly became very quickly obvious that they'd been blinded by a smoke screen. They didn't believe me then and they still don't now. So things that you think are great may be you believing something she knows not to be true.