THAT MIGHT SEEM ODD. As a sensei, after all, you’re supposed to know. That’s why you’re a sensei, isn’t it? If you signed up for a French class and on the first day you asked your teacher how to say “hello” in French and he said, “I don’t know,” you’d probably take steps to get your tuition refunded.
So, yes, if you don’t know the basics of your art and how to teach them, you shouldn’t be running a class. If, however, you have students who have trained for 10 or 20 years and you’ve raised them to high levels of competency, then “I don’t know” is something they should hear, at least from time to time. And if it’s in response to a question from a beginner, answering with…