Instead of telling them to bend their arms, tell them to bring their hips closer to their elbows. This will keep their body position correct. Knees should be bent close to 90 degrees. You are going to find that as a student gets tired, their legs get stiffer and straighter. Straight legs make for hard falls, a lot of spray, and poor body position.
Short sessions are the best approach.
As soon as you see a student begin to fatigue, you end the session. It'll keep the falls to a minimum, the body postion good, and the learning fun. It seems by the time the students gets feet forward on the kneebard, they're too tired to maintain good body position. The boom is a great learning tool. It allows the student to 'cheat' because they have something solid to hang on to.