Teen Idol by Meg Cabot
Jenny is the girl next door type—level headed, well-liked, friendly to everyone, and a natural peacemaker. But she’s more than a little surprised when school administrators ask her to keep their secret. Luke Striker, teen movie star, wants to enroll in her school to experience a "regular high school" for an upcoming role, and they want Jenny to be his mentor. While Jenny can be trusted not to expose the secret, can she handle the overly critical Luke and his harsh observations? There’s no danger of Jenny falling for Luke since he’s the one person who can make her really mad. Yet Luke does make a lasting impression and challenges Jenny to take a leadership role in making the school better for everyone.  Can one teen really do that?

Meg Cabot’s website has an excerpt from the book and a personal blog that you might want to read.