in an ideal world, yes, primary school teachers would be all over it. But I find maths quite comfortable. My grammar, sentence structure and knowledge of fronted adverbials on the other hand is crap.
I suppose I could make a massive fuss, embarrass the teacher and make sure the children think it's important to make the most of someone being wrong. I'd rather engage constructively and hope that it helps the teacher as well as making sure the class are taught better.
And in the meantime, I really appreciate that my child is learning that it's OK to make mistakes, be challenged to think again and learn from it. Not just by being told but by seeing their teacher have the humility and teachability to do it themselves.