My instructor is quite straightforward and will tell participants directly (me included) when they are doing something unsafe or wrong. My problem is that my knees tend to be too close together. So he calls me on it. He calls out other people in the class to, among other things, increase resistance to stop bouncing, ride properly with your hands on the bars, decrease resistance to get up to cadence, chin up to breathe properly, drink up, etc. It's his job to teach us what's right. I expect that, and would not like it if he just kept quiet or made general comments to the group. Some people don't know they are doing things wrong, and some just need reminding. He's good at learning people's names, and at the beginning of the class will ask people he doesn't know what their names are. I think people behave better since he knows their names. There's no chitchat, but there is a friendly banter between the instructor and the participants.

Some people show up really early and put their towel on the bike and then go do their core work and come back at the beginning of class. That seems perfectly acceptable to me. It's hard to differentiate between those towels and ones that other people have put up for friends. However, it's reasonable to make it clear that five minutes before class the people waiting get dibs on unoccupied bikes. Put up signs a few weeks before you start this so that everyone knows. It'll only take a few times for things to work themselves out.