The Red Rock Mysteries Series by Jerry B. Jenkins, Chris Fabry

Bryce and Ashley Timberline are normal 13-year-old twins, except for one thing—they discover action-packed mystery wherever they go. Whether it's searching for a missing writer or fearing for their lives at the bottom of a canyon, Bryce and Ashley never lose their taste for adventure. Wanting to get to the bottom of any mystery, these twins find themselves on a nonstop search for the truth.

When eighth grade begins, twins Ashley and Bryce get caught up in investigating why their principal left under mysterious circumstances, even as they struggle to get along with his replacement, who seems to be out to get them.