For the past few years, I knew which semester the students were taking P.E. For several months, they would waddle slowly into class and groan as they tried to sit down. "My butt hurts" ... "Ow" ... "50 burpees!" ... "walking lunges!" They'd seemingly "complain" about the PE teacher, and in a wide-eyed manner, regale others with astonishment at what difficult physical feats they were required to perform. This would happen basically all semester, whichever kid, whichever semester they had P.E.
Fast forward to this year. That teacher is still around, but some of the students now have a new P.E. teacher. This is what one kid told me after a few weeks had passed: We've asked her to work us harder. We want her to be tough like Coach C. She DID ramp it up a bit, but it's still not as challenging.
Maybe what we hear as complaints is just a badge of honor that the kids are touting as they process in wonder at what hard things they can do when they are "forced" to. They are saying it out loud as sort of a "go me" type of conversational pat on the back. They complain, because they are humans and teens, but secretly they crave the challenge.
So maybe when we hear that they have SO much homework and the problems are SO hard, then we should reply, "yes, and look at how great you are doing by struggling and keeping at it". Work those brain muscles!