I've got some advice for ya, take it or leave it.... I actually competed in some O-lifting in college, and worked at YMCA , where ironically, I started to teach aerobics (step, spin, the whole gammet)........... you don't even have to really ride the bike, you can just teach the class. Pedaling cadence can "look" fast or slower depending on leg length so mention to the class not to look at someone else's pedal speed to judge how fast they should be pedaling b/c
1) their leg lenth may differ
2) lots of morons who attend spin class have practically no tension and pedal like a bat out of hell, thinking that the faster they pedal the better. I read a Poliquin article that ripped on spinning "what good would it be for your tricep to swat a fly swatter as fast as you can 100 times?" He's got an article on how spinning makes you fat............
3)definitely do intervals! typically people wouldnt' do them on their own so you can use the interval training to take their conditioning to a new level.
4)go www.sfresources.com, register, and go to class desings. There are pre made spin classes, with intervals galore, tells ya exactly what to do.
5)If they are working hard enough, they shouldn't even notice you're not really working out, besides the workout is really for them, not for you right? (and mention that, they will feel that you have their best interest in mind!)
6)not a big fan of spin myself, but it is a class anyone can join without being smacked around if your new as your the one in control.
7) I try to stay away from telling people they should be at 80% or 90% or 50% (etc.) because they don't really know what 80% REALLY means. So I just try to use RPE and let them know they are in the recovery for the next minute,they should feel like they could maintain that level for an hour and their breathing should be more from the belly or they are in a work part and their breathing will become more rapid, from the chest, converstation could be maintained for 2 sentences, etc.
Hope that helps a bit.