You miss the point. He isn't really there to "teach" them but to observe who is paying the most attention to his classes - which is considered a privilege due to the knowledge it provides, and who isn't paying attention i.e squandering the opportunity. This gives the grandfather some insight to who has the most potential.
The ones who should be providing "guidance" is the parents not him. And based on what he observes by how they behave it also gives the grandfather a preview of how they have been raised. Bellesac being a prime example of a child that that is stupid and arrogant and has been raised by parents with similar attributes.
I think that mr. Yello haired guy does not have efficient methods in teaching. I think that they are still children who needs proper guidance barely 8 and 10 years old. Not correcting them from what is right and wrong, is like letting them be a fool for their life. I don't mind if his methods are on high school students and college students.