I don't think he should have done, but not any grounds to do with freedom of expression. Teachers are not there to teach as effectively as they can, not to freely express themselves. They have to pretend to like Jane Eyre, that they never take drugs and that it isn't funny to take the piss out of the head of geography, for example. If they want freedom of expression, they can always take up performance art in the summer holidays.
Basically, if your teaching doesn't actively look for ways to avoid dividing and alienating students, then it's poor teaching. That doesn't by any means imply that tough topics should not be taught, but how should be thought about in detail. I think any lesson that actually plans in the self-exclusion of some students while the rest of the class discuss issues directly related to their place in society sounds, frankly, fucking attrotious.