“First-amendment-protected speech” is everything you say every day unless you like to make death threats or hire people to commit crimes. “How’s the weather” is first amendment protected speech.
We would not be having this conversation if his “first amendment protected speech” was loudly detailing his sex life in front of the teachers or if he was displaying his artwork that depicted realistic grizzly scenes of dismembered bodies. Those would be first amendment protected speech, but it would also be a damn obvious reason why you wouldn’t bring school groups to his farm anymore.
It’s not just racism that’s protected by the first amendment.
But that’s how it looks to an outsider. Honestly, whenever people bring up the first amendment it is pretty much guaranteed we are talking about racist speech. It’s getting to be a dog whistle.
You already know that there are things that a person could say - things that are not illegal and that could not be made illegal because of the first amendment - that would be completely justifiable reasons for the school to stop going to their business. The only confusion comes from the belief that racism is a special protected category of speech. That’s malware the racists have implanted in your head. Delete it.