Nice thought, but it won't work. That will just leave her trying to explain it's okay, she doesn't mind if he needs to spend time studying, she can help take care of him so he can focus on his studies more, etc. Anything other than the point-blank, "Sorry it was a mistake, there's someone else I'm seeing" explanation leaves things open to misunderstanding and just enough uncertainty for her to think there may still be something to salvage. Don't string her along. If it was only one date, as Andromeda pointed out, she'll get over it as long as you make it clear that nothing more is going to come of it.
Of course, there is the slightly softer approach where you don't have to admit to being more interested in someone else, which is pretty harsh to hear. Just keep it simple: "I'm sorry, but I've come to realize we just don't click (you can substitute a number of things for 'don't click': 'aren't right for each other,' 'aren't going to work out as a couple,' 'didn't have the right chemistry'). I don't want to hurt you by pretending there's something that isn't there and stringing you along." This is actually a bit better than telling her you never should have gone out with her in the first place, which she could take as a huge insult, or might leave her feeling used. There's no rule that you can't date more than one person at a time when trying to find one to build a relationship with. That's the point of dating, to find the right one, not to instantly settle down with the first one you come across, so it shouldn't be an issue if you appear to have quickly begun dating someone else.
