I'm kinda surprised that the instructor doesn't go over to them and correct their position. Even though it's harder to do then say in a step class. A good instructor should make sure that the students are using correct form to avoid injuries. I used to teach step (WAYYY BACK) and I would actually go to peoples steps as I was instructing the class and give them cues. I would also try to pull them aside at the end and explain what I was trying to point out and why.
Our gym has numbers on the bikes and the spin instructor will say "bike #3 flatten back" etc. Maybe thats what you need at your gym??