I have had both my teachers from high school and university sit down whenever they were teaching and there wasn't any kind of inquiries on them that they shouldn't sit down while teaching. I have been a teacher too and I sit down whenever I want whenever I'm in class.
A teacher sitting down while teaching won't be an issue as long as they are sitting at a position where the whole class can see them. This is because there are times during teaching that the teacher will have to demonstrate.