There have been so many times when my students (including my own children) have asked if they can play ahead in the lesson books. "I can play them, mom, I know I can" or, "they're so easy, can I play through the end of the book". In fact, I heard that so much that I had to develop some "rules" about playing ahead.
> If you look ahead in the book, you may only play through a song once or twice. This is so they don't form a habit of playing it their way.
> When we get to a song/lesson that you have played through once or twice, you have to be willing to listen to my explanation of the song/lesson AND understand that you might have played it incorrectly on your own. I will help you play it correctly.
If a student keeps asking to look ahead, it might be a sign that the practice load is not enough to challenge them during the week. This is a great time to bring in supplemental music. Some of my favorite supplements are Dozen A Day--which is a great finger exercise book, and also the Piano Adventures Popular and Performance books that match the level the student is at.
Keeping students challenged but not overwhelmed is a delicate balance, but one that brings about great accomplishments!