Any accommodation which requires a workplace accepts anti-trans hate or bigotry is by definition a substantial burden.

No workplace should be required to put up with shitheads nor scramble to counteract the harm they do to their guests, customers, and students, be they trans themself or simply decent people.

While I suppose the teacher could find a different job if it bothers them, I personally believe this teacher (and all transphobes) should be made to "say the pronouns!" at gunpoint, irrespective where they work (like that inspirational meme the right so oft posts).