If you think you are spinning too fast, then slow down. Hard to do with the music, but is worth an attempt. The other thing is leg and foot fit. Saddle position is easy to set up for the legs, but the toe in or out-- Should be toe in really- is very difficult to adjust for. I have the straps on this really tight so that my feet do not pull off of the pedals, and this makes me realise why I have SPD's. I have a fair amount of toe in on my bike, and can never get it right on the spinner bikes.

The other thing is to see if the instructor can do the majority of a class as if going uphill. Lower cadence and more pressure on the wheel. My instructor worked that one on us last year, and we found who the hill climbers are. He kept putting the pressure on and at one point made us get off the bike and stretch. He then checked the pressure on the wheel to see if anyone was cheating- Luckily none were.