DS is just 4. At swimming today there was a different teacher to usual. The teacher goes into the pool with 4 children. The teacher was holding each child's head to get them to swim a width kicking their legs on their backs. When it was DS's turn he was clearly uncomfortable and tried to stand up after a few kicks, but the teacher put him back into position. DS then panicked, screamed out and tried to stand up. The teacher kept putting him back onto his back, holding his head with her hands. By the time they got to the end of the width DS was crying.
From the viewing area I could see and hear DS crying while the other children took their turn. The teacher then made DS do the same again, even though he resisted, cried and panicked again. Mid length she moved him to resting his head in her shoulders but he was still scared. She didn't seem to be doing much to comfort him. Then they did a third width, again with DS upset.
I went down to poolside but a second teacher told me he was fine and to go back upstairs 
For the rest of the class DS was visibly crying and calling for mummy, so approx 15 mins. He cheered up in the fun activity at the end. When the teacher bought the children to the changing room I said that I was uncomfortable about DS being forced to do something when he was obviously terrified. She said he was fine and the busied herself with the next group of children.
DS clung to me and cried while I dressed him.
To contextualise, this is his 6th lesson ever and the usual teacher knew that he is a nervous child and reluctant to swim on his back.
AIBU to think the replacement teacher was inappropriate to force DS to swim on his back when he was clearly upset? Would I be out of order to email the regular teacher (who is the owner of the swim school) to express my concerns?