well, you can't tell her 'we' should wait. because that's condescending. what -she_ should do, is entirely up to her. and it sounds like she's made up her mind as to what she thinks that is you can't go and undermine her integrity just because _you_ have reservations.
1. its perfectly acceptable to say you do not want to be in a relationship whilst finishing school.
2. I'd leave out the lie about thinking youd have more time after and it isn't fair to her now blablabla. since you already said that you're not sure how you feel about her.
again, it is not your choice as to what is fair to her. that's her choice. maybe she doesn't need much time at all with a boyfriend. you can't just go telling her what you think is best for her when you don't even know what it is she wants, vs what you an provide.
anyways as you said you're undecided. d stick with that as its about you. not her. speak on what you know-yourself. and never tell people what they should do. unless they ask.