These Planks are simple and do the trick. You will need a step for this exercise. Get in the push up position with elbows on mat and toes on step. Suck your belly button in to your spine, & get that but tight so you don't spill that imaginary glass of juice on your butt!
Add this to the end of your ab class for final torture.
P.S. check out my other ab routines, and let me know what your class thinks after trying them, I like feedback, it helps me stay on my game.