I get these from so many people too many times already:
"You don't look like a Math teacher."
"You look too young to be a Math teacher."
"Really? Math teacher ah? So young!"
"You don't look like someone who knows Math."
"I thought you were Chinese because you teach Math. But you don't look Chinese. What's your name?"
"Are you sure you can teach Math?"
What do you think? Does this face look like someone who doesn't teach math?