Say It With Food
Asking a girl to be your valentine with the help of food is a creative way to get her attention. One approach is to visit a bakery, buy a large cookie and have "Will you be my valentine?" written on it in icing. If you enjoy eating out together -- whether at the school cafeteria or the mall food court -- buy a pizza and then quickly write your question on the inside of the lid before presenting it to her. In the same manner, a quick message on a slip of paper on her cafeteria tray -- provided she accepts your offer to have lunch with you -- can be a pleasant surprise.
A "Note"-Worthy Approach
Giving her a sincerely written note that expresses your feelings for her and asks her to be your valentine never goes out of style, but you can add fun to this approach by giving her the note in creative ways. Slip the note inside a balloon and leave it taped to the exterior of her locker. She'll have to get the nerve to pop the balloon to read your note, so leave a sign of the note within. If she drives, find her car in the parking lot and leave your note under a windshield wiper. She might initially think she's received a parking ticket, but she will have a fun surprise upon reading your note.
A Musical Interlude
If you know the girl enjoys music, suggest listening to a few songs during lunch or on your break at work. Make a short recording of you asking her to be your valentine, and play this recording after a couple of songs you think she'll enjoy. For a fun twist, pretend to be a radio DJ on your recording. For example, say, "We'll have more music in a moment, but first, Alex wants to know if Sarah will be his valentine." This method is especially effective if you're nervous about popping the question in person.
Leave Her Hanging
Play a game of hangman with the special girl as a creative way to pop the question. Hangman games typically focus around a single word, but when you write spaces for the phrase "Will you be my valentine?" she'll be curious to identify a word that seems so lengthy. Because your question contains several letters, she shouldn't have trouble revealing the letters one by one until your question appears right in front of her.