When intro-ing my 12 yr olds, I had them keep it in the small ring, too. I also made sure to have them think about how fast their feet were going around. I tried to keep the focus off gears/gearing. I only mention if they were pedaling slowly, then shift to an easier gear, and vice-versa. Then, I explain why using 2 muscles in the body: legs and heart/lungs. The heart/lungs will last longer. The legs, while stronger for a few seconds, will tire out. Last, they will enjoy riding much more (it will be easier) if they focus on heart/lungs by spinning their feet faster. It takes awhile, but they've gotten it so far.