To really blast the inner thigh and abs try this progressive move:
Position 1: Get class to lie on their backs with the ball between their legs and simply squeeze the ball with their knees.
Position 2: Ask them to lift their feet off the floor raising the ball still between the knees and squeeze here, working the inner thigh.
Position 3: After 3 squeezes and 1 rest slowly lower the ball towards the floor for the count of 4 then raise again before their heels touch the floor. Push the back into the floor and feel that burn on the abs.
Repeat as many times as they are capable of doing (I usually find 6-8 is plenty)