Keeping in touch with teacher after sixth form?
There's no reason why you shouldn't ask if she would like to do that, but be prepared for her to say no; teachers tend to keep even former students at arm's length, for a variety of reasons.
While I tend to agree with bluebellbkk it would be much better if you waited a year or two after you have finished school. Right now, if you stop to think about it what would and your teacher talk about? Teachers don't always want to talk about school when they are on their own time. Why not hand write a thank you note expressing your appreciation?
That's up to her. If she wants to keep in touch with you, that's great. My grandmother was a high school principal for 22 years. Although it's been a number if years when I am in my mothers home town I had people driving by asking me if I am my grand mother's grand daughter and I look like my mother. It is a small town. She was your mentor. She can still be your mentor. But it's up to her if she wants to stay in contact.
I don't think it's too odd, she may decline if she feels it's unprofessional, but I'd express how much you admire her and will miss her. I mean it's not a date or anything!
She wont be allowed to meet up for coffee and it wont be long before you forget about her so move on and do nothing.