Well, for me it would be a big deception if that happened to me, provided that the students didn't actually have reasons to complain. If so, avoid teaching this "extra material":
- If the students do not have the required previous knowledge. Make sure they all have it (or at least the big majority). Having been taught before doesn't mean they remember it either.
- If the students have so much pressure. Nobody appreciates learning when they've got more urgent things.
- If those contents are quite unrelated to the main topic, while other issues are more important. I remember things like "why are you teaching us group theory when we are needing Fourier transforms and nobody's helpping us?". That's very dissapointing.
So, if it is not any of those cases, or similar, then yes, the situation must change. I assume we're talking about lazy / unconcious students who just want to pass, and they don't really care about learning or not.
So here my advice. In short, it is: "okay, you won't need it for my exam, but next year you'll be using this. The teacher won't have time to explain this, and (s)he will go very fast through this, so you better understand it now so that you can follow he class".
But it is important that you must transmit how much one suffers when one gets lost in class. The suffering of being in your seat and ask yourself "what am I doing here?" And you can't leave. That's stressing and agonizing. And not only you have a bad time in class. Then you don't understand anything. Since you don't udnerstand it, lessons go by and you keep at the start point, so you can lose a whole year. Make them imagine and "live" the situation. That's what I'd do.
EDIT: I didn't have much time and I left the answer like this. Now, re-reading it looks quite terrible . Of course, you should also do the opposite: transmit how satisfactoy is to learn something new, which helps people make sense of everything. Connecting concepts is always good. Those times when "suddenly everything makes sense" are enormously satisfying. You must transmit all this as well.