I agree with the core of Abby's too-brief advice, which (I think) is that it's not necessary for the LW to directly say, "I'd like to be friends and maybe more." The LW should just be friendly, chat with the boy in school, go to some of the same events, get to know him, and let him get to know her. I don't see the need for an intro text, but then, I'm not a teenager. I graduated from high school 17 years ago, so my perspective here may not be relevant.