There was a new dog in class (he'd been getting neutered the week before). Junior is a Brittany who took an extreme interest in Thor. Thor was equally interested, and the two kept trying to get closer to each other. I sat out several exercises in order to work on attention on the sidelines with Thor. He responded well to this, and I will plan to do it again.
After we rejoined the class, Junior managed to get away from his (two) handlers and came right to Thor. Junior growled, Thor play bowed. It was a little bit scary because of the growl. After Junior was corraled, Kathy used the opportunity to explain why a dog should never be corrected for growling. Corrections may remove the growling and you are left with a dog who gives no warning. This can be a very dangerous dog. This is something I understand well. No doubt my Vada was punished for growling. Her attacks (of other dogs) came with no indication of how she was feeling. Eventually, Vada became a home dog and I no longer took her places because of her unpredictability. She was much happier after that.
I told Junior's owners that I saw nothing really alarming in his behavior and that Kathy was right. However, they decided not to return to class. They emailed Kathy and explained that they had moved and it was simply too far to come. I suspect they were surprised at how much difficulty they had controlling Junior and that that played at least some part in their decision. Anyway, I'm glad they won't be back.