I think the English translation should be "When I said that the teacher didn't know what he was talking about, I was sent home".
It's slightly more natural, but the way it currently is isn't incorrect, and it makes it easier for learners when they come across the reverse exercise (English to Hebrew).
Actually I think it is incorrect. But I got it right the first time without thinking about it so maybe its worth keeping anyway.
The use of the present tense "doesn't know" instead of "didn't know" implies that the student thinks the teacher is wrong most of the time, not just on that one occasion.
Should also accept the got passive "I got sent home".
I said "what he is speaking about" instead of "what he's talking about" and was marked wrong. That is an equally valid translation. Reported 7/24/20.
That's pretty cheeky!
It tells me I have a typo -- but the word they want "NOT" wasn't in the options. Please correct.