Great articles and sites for building core strength!  I will definitely be adding some of these exercises to my teen and tween jazz classes (and my own personal workouts).

4 Great Core Exercises:  Click on the title of each exercise to see a video demonstration.

Max Capacity Training: Designed for body builders, this site has a bunch of great exercises for strengthening your core (and arms.. and legs…).  There are 12 exercises per week and exercises change every three weeks, so click on Weeks 1, 4, 7 and 10 to see all the exercises.