OMG
First, the photos. How rude! Really, do that on your own time when someone is not paying you for instruction during a class. That is so unprofessional.
Second, if that would have been my instructor she would have chucked that Rott right out of class. These are the things that happen when you are unprofessional and don't have control of your class. The first time it happen my instructor would have had a very frank discussion with the owner. The second they are gone. There would never be a third time. She would do privates but never endanger other dogs in class. She has BC as her breed but her husband is a retire K9 officer and they had had a Rott, a couple GSD and a couple Mals so does know the big guarding breeds and has little tolerance for owner denial. A couple months ago,she kicked out a huge Mastiff who was a distaster waiting to happen. The dog bit the husband and tried to bite another dog in class and her(the instructor) on the hand as he passed by. She told them the dog was dangerous. They were p**sed and in total denial. A week later, at home the wife was sitting behind the dog petting it long the back and he suddenly swung around and mauled her throat. She was lucky she didn't die. Warning signs everywhere, no one listened.
I think the freeze behavior was Ellie on high alert. Waiting for the next assult kind of thing. I have no doubt that if she was pushed she would have defended herself with a bite. I have seen dogs in the vet clinic do this. It means back off or I will bite you. When A dog freezes and stiffens and you don't have good control of it you are in trouble. At that point, I slowly disengage and move slowly out of the way. The bite is a hair trigger away and anything can set it off. Once the danger passed Ellie retrned to normal.
I would demand my money back and never return. I hope you can find another class for you and Ellie. I went to 4 different schools before I found the one I go to now. At one of them, I really did not feel safe. Too many out of control dogs. I was on pin and needles the whole time. One night Justice almost got bitten by a GSD and the instructor didn't seem to concerned. The next week another GSD went for another dog again no real concern. The next week more lunging/growling behavior. Finally one week, the first GSD attacked another dog on a sit stay, the second GSD joined in the fight, the rest of the dogs scattered. Justice was only 1 dog away when this went down. I never returned. My dogs safety is too important. The school I go to now I have witnessed less aggression in 8 yrs then I did in 6 weeks at the other place. Comes down to instructor control and commitment to providing a safe environment.