In high school substitutes were rarely treated like the full time teachers. Students disrespected them and were sometimes outright rude to them.
I was never able to understand why someone would want to be a substitute.
Part of getting started in the Spinning world typically involves becoming a sub. Usually the process of instructing starts with becoming a sub, then once you have "proved yourself" you can audition for a class of your own.
I'm happy to say that subbing a spinning class is NOTHING like subbing a class in school.
I'll admit though that i've showed up to a spinning class (as a student), seen that the regular instructor wasn't there and toyed with leaving. Given this knowledge it's my personal mission to make those who stay happy with their decision.
Subbing is good practice. However, it can be hard. You're walking in to a group you don't know, with preferences you don't know, and often times, they're watching you with a critical eye.
Some suggestions (from experience) are to contact the instructor you are covering for to find out the class's likes and dislikes as well as how he/she usually runs the show. Walk in armed with this knowledge and confident and you're sure to do well.
I'm subbing for a friend tonight, but having taken his class for over two years I know exactly what i'm walking in to, and love it!