A: This is a fairly common problem. Try this exercise to help students learn how to move their legs while maintaining abdominal engagement.
Begin by having them lie on their back with their knees bent, feet resting on the floor and spine lengthened. Then, have them softly flex at the hips and lift each foot off the ground toward the chest, without tucking the pelvis under or pushing the low back against the floor. This is the starting position.
Keeping one knee bent, have them slowly take the other leg away from their chest, staying parallel to the floor. (Monitor their pelvis, and ask them to stop if their lower back starts to arch away from the floor.) Have them bring the extended leg back in toward the chest as they switch legs and begin to extend the other leg.
Alternate the legs slowly, and if you want their abs to work harder, don't have them bring the knee as far in toward the chest. This will keep the abdominal muscles more engaged. Remind your students to keep breathing when they use those deep abdominals.
During this exercise, the upper body, neck, jaws and arms are relaxed. You can add in a half sit-up or some twists as you have them move the legs, if you want them to challenge the abdominals even more.