I'd go up to her at some point just before she's due to go to her locker and, as casually as possible, ask her if she's seen some valuable page of notes/assignment or homework sheet that you have lost. If she asks why you're asking her just say you last saw it near your locker and thought since she's close by she might have seen something. Anyway, just keep chatting small talk about how annoying it is when people pinch stuff until she opens her locker, and voila- inside her locker is your sheet of notes, slipped under the door earlier by *someone*... (that's you by the way). Pick it up and give her a sly grin and start playfully accusing her of stealing it herself. She'll either be astonished that it was there and be focused on proving her innocence to you, or will figure out it was you and accuse you of doing it just to talk with her.
Whichever it is, just hear none of it and keep playfully accusing her of doing it just to talk with you as you slip it into a folder and get ready to go to a class (real or imagined). Cut her off at some point and casually say that you have to get to class, but that the matter ain't over. Make sure you say all that with plenty of tongue in cheek, as if even if she had done it you didn't really mind. Make your exit, and she'll be thinking about you for the rest of the day.
Better still, next time you're both at your lockers you're pretty much guaranteed that she'll strike up a conversation with you, either to continue pleading her innocence or continue labeling you as the culprit. If she's a bit shy about doing so just flash her that sly smile without saying anything when you're both at your lockers. Throw in a slow head shake as you return focus back to your locker too and it should do the trick. Continue to pretend it's all a ploy from her with plenty of 'yeah, yeah, yeahs' etc.
You can either wait through several of these encounters to ease into a date proposition, which is fun but risks her putting you in the friend zone, or you can grab the bull by the horns and on the second or third (but NOT the first!) such conversation just say (again casually) "Alright, how about we sort this out once and for all over coffee/lunch/dinner sometime? Which you ask depends on the girl really, just make sure it's something away from school with just the two of you. After that you'll be cruising.
Let me know if this proves helpful. Good luck, mate!