So, after getting some more information, what we have been experiencing may have been an extinction burst related to stuff which has been going on at school.
Extinction bursts happen when you try to eliminate or change a behaviour. Any behaviour that a child (or really anyone) does on a regular basis is working for them in some way. A kid whose parents cave every time they throw a tantrum learns that throwing a tantrum is a pretty good way to get what they want. Even if it's only sometimes, that can be enough to reinforce the tantrum as a good thing to try.
So when parents change the environment and expectations to make a behaviour not work, the child doesn't automatically say "oh well, guess that doesn't work anymore." Instead he or she will escalate the behaviour to make it work again. So in the tantrum example, the kid may throw more tantrums or bigger tantrums (or both). That's the extinction burst.
This is where it gets tricky. If the parent gets worn out and caves, even once, the extinction burst will drag on and continue to escalate. So, as exhausting as it is, parents or teachers have to stand firm. The more consistent they are, the faster the behaviour will actually disappear.
There had been some confusion at school over what constituted "acknowledging" Alex's headbanging. When I told them not to acknowledge it at all, I meant to say that as far as teacher reactions go, they need to act as if it doesn't exist and it didn't happen. Instead, he was being told "Don't do that" which prompted him to begin using it for attention and led to an increase in the behaviour.
I'm hoping that we're back on track now. We've talked to them about not rewarding him for not head-banging (since that acknowledges the behaviour, instead we've suggested rewarding him for cheerful cooperation). We've asked them to make sure that substitutes and co-op students are aware not to say anything or intervene.
But the last few days have likely been an extinction burst where Alex was trying to get his head-banging to work again. I didn't consider that as I thought we'd gone through this process back in December but apparently not quite. If we are moving toward consistency, then hopefully it will all be over soon.