It’s the school district’s responsibility to both protect their students and ensure inclusion of all students in their official programs. If even a small proportion of the student population (say, African Americans) have a reasonable reason to fear going to a farm owned by a guy who’s made publicly racist statements, where warfare re-enactments are going on, then they have a responsibility to cancel the trip.
I’m not African American and I’d hesitate to go there with the things this jerkface has written on Twitter. If I were African American, I’d be playing out scenes from Get Out in my head on the bus ride over. Nopenopenope!