Use a Polaroid camera and take a photo of yourself holding up a sign that says "Yes." Choosing a charming or unique outfit that suits your personality and the location you choose is important when trying to take a good photo. You can include friends or choose a place connected with you and your date. The photo can be decorated, especially along the white border. Once it is taken you can put it into an envelope and slip it into their locker, or have a mutual friend give it to them at an unexpected moment.
Glow in the Dark Stickers
Attach glow in the dark stickers to their backpack or one of their book covers you know they’ll be taking home with them. When they turn off the light they will suddenly see the message. This has the advantage of an unexpected and magical effect. You need only to be careful that they will actually see the message, so only try this if you know them well enough to know it will work.
Poetry has a long tradition of romantic use for a good reason. A beautiful poem is a memorable, striking way to combine your reply with a sense of your regard for the person who has asked you. A simple rhyme or a sonnet are tried and true forms, and examples for inspiration abound, such as poetry by Pablo Neruda, William Shakespeare and E.B. White.
Leave an origami crane, flower, box or any other creative design on their desk or on their books when they are out of the room. This should include a message for them written along the side. The striking object and its colorful paper can’t fail to catch their attention and leave them with an art object by which they can long remember the moment.