Teen Idol by Meg Cabot
Jenny Greenley is the kind of girl who everyone at school likes. She is, as she says, mayonnaise. She keeps everything together. Jenny is so trustworthy, that she is the top secret advice columnist for the school paper and is also chosen for a top secret mission to show around a "new" student who is really teen heartthrob Luke Striker incognito.
Another fun read from Meg Cabot!