From a girls point of view how should I ask her to the movies?

FIrst off I have decided to take the plunge and ask a girl who Ive liked for a while out, the only problem is shes been a good friend of mine for a couple years now, so it makes it hard to tell if shes interested or not. Basically Im debating whether I should ask her in person, but I might only have a few seconds in school to do so otherwise i wont see her alone. Or I could leave a letter for her somewhere of some kind asking/ leave her a text message. I know people say ask in person but ill end up just cutting straight to the point, whereas I could leave a little more info, and let her think about it without uncomfortable puttin her on the spot. WHich of these three do you think seems best?