I tend to think the role of a teacher is to nurture students to be 'brighter' than them. That is how teachers who barely got C's are able to produce kids with A's who ends up becoming doctors, engineers etc. In fact strictly speaking, a teacher should only provoke their students' thinking instead coaching them how to think. In that case, the teacher need not be 'brighter' than the pupils, they only need to be trained how to teach.
The best teachers are by the way not the brightest. There are very intelligent and bright people who cant even teach someone anything. Yet there are others who are not sooo bright but are very good in nurturing pupils/students to think beyond boundaries.
I also think it depends on what the teacher is good at. I know there are Wazuans who can be beaten hands down by their sons/daughters if they sat in a Kiswahili exam together.
What am I saying?