This is what I get for never using Japanese in class

It's a year 3 class at elementary school, and the lesson's topic is jobs, and the structure "I'm a ~." I open with an example.

"I'm a teacher. Matsumoto-sensei [the classroom teacher] is a teacher too."

Boy at the front: eh! He spoke Japanese!

Apparently, I shouldn't say people's names in Japanese. I should translate the kanji.
"Pine-origin-teacher is..."