Ok, this is what you do.
You go up to her boyfriend and go: Psst... <insert hottest girl in your grade here> likes you, but she doesn't ask you because you are going out with Erica.
Then, when the shallow b*stard breaks up with Erica, you pull off some sweet break dance moves in the talent show and at the end go: I dedicated this to Erica who is the sweetest, smartest, most beautiful girl in the world.
If you can't dance then like... um... fix some computers and dedicate that.