Thought I would not argue at the philosophicall level. I would also not aim the argument at the teacher (when I am calm).
I would stick to the facts, showing the teacher why your son's method is valid and why make the false prediction is wasting everybody's time--If that's true.
There is a typo in the last paragraph that needs to be corrected if you want to send it:
"you still answer kids to follow the exactly way you think is the right way to get answer."
should be
"you still ask kids to follow the exactly way you think is the right way to get answer."