Best to do so is after you graduated or at least when you know what your grade classification is before you "fess up" to her
If she is a stunning lady, most of them would have had a lot of admirers from undergrad / grads or other colleagues expressing their fondness to her anyway. You simply become just another name on a very long list of people.
it depends on how you want the relationship to develop - especially now that you told me that you would be studying for another year and she will be grading your papers. Sometimes life is easier when you dont put things in black and white - and I think if you are really into her and want to convey a message that you are fond of her, there are many other ways apart from doing it face to face. You can flirt with her (maybe) and maybe ask a hypothetical question to test her reaction, either way.
I have had a reverse experience myself when i was in highschool with a female teacher - I could feel the intensity of her love to me and that was a shock to me that emotions can be this intense. Afterwards I got into uni, I also had a similar experience with a female faculty member and I would have to say, she was much more restrained and professional. I left uni and because I did not know how to handle the situation, I went quiet for a few years and only recently did I reconnect with her and I hope that she has forgiven me about that.
I think at tertiary level, relationships should be more liberal provided that both parties are mature and know what lines cannot be crossed.
As much as it is considered unprofessional, we should also realise that a lot of academics spend most of their time at work place and the people that they are surrounded with are often also from within, which means very little time tor opportunity to socialize with the outside world and I sympathize them for that.
At the end of the day, a woman in academia is fundamentally no different from a woman from a coporate or a supermarket. If they give you their heart you for real love are expected to treat her with equal gentlemen etiquette.