Talk to one of your teachers and ask if he will help you with your homecoming proposal. Have your teacher hand out a fake pop quiz. Everyone except your boyfriend will get a piece of paper telling them that there is no quiz and to sit quiet. Have your teacher give your boyfriend a quiz with very difficult problems, and the last problem should read, "This quiz won't count if you go to homecoming with _____." This will surprise him and give him a good laugh.
Go over to your boyfriend's house when he is not home and write "Homecoming?" in chalk on his driveway or sidewalk. Use different colors to decorate and make it stand out. Make sure it is okay with his parents first. Find out when he will be coming home, and wait outside so you can see his reaction and get an answer.
If you have a little money to throw around, rent out a large billboard that is on his route to school or see if your high school will let you post your message on the marquee. Write something cute like "John Smith, Will You Be My Date For Homecoming?" If he can't drive, talk to his parents and make sure they drive by the billboard and slyly point it out if he doesn't notice.
Make your boyfriend a mixed CD with all of his favorite songs, and include some that both of you like. Record a cute message asking him to homecoming. You can use a simple program like Garage Band or a voice recorder. Many cell phones have voice recorders as well. Upload the track to your iTunes library and make it the last track of the CD. Leave a note inside the CD case telling your boyfriend to listen to the last track and let you know what he thinks.