Thinking back to when I was in highschool, if I was the recepient of a similar gesture, I would spend all my energy grilling my friends about which one of them was pulling a prank on me. If I figured out it was for real, I'd probably imagine it was someone I already liked. If I found out it was someone I barely knew, even if he seemed interesting and attractive, I would probably avoid him because I'd have no idea how to react.
Actually, I think I have been in a situation in your sort of place before. In junior high, I expressed my affection for a boy i liked by making him cards and crap like that. I was incapable of having a normal conversation with him so I felt the 'get to know you' approach was not an option I believe he was flattered but felt extremely awkward about the whole thing, as did I. He liked to brag to his friends that I was stalking him.
Another point: Whatever you do, she WILL tell her friends all about it. If its just spoken words, they will likely disinigrate quickly from people's minds. If you say the wrong thing, you always have another chance. But anonymous love notes are kept forever, read out of context, and will almost certainly be mocked profusely at the next slumber party.
I understand where you're coming from, because when you like someone and have week social skills, particularly when you're young, it seems easier to make some anonymous but gallant gesture and hope for your feelings to be returned. Talking seems like the hardest thing, but it is actually the best. Even if you sound all awkward and it's obvious she makes you flustered, attempting conversation will be better for you in the long run. She might think it's cute. Good luck !