Being academically smart or the smartest... has nothing to do with emotional maturity or development.
My son has a classmate, that is very bright. But, he is also known for causing a lot of trouble.... to the Teachers and to the other kids.
So then, the classroom and the other kids, gets affected. And that is where, it becomes a "problem." Other kids, complain about him too, because he pointedly, causes trouble, to others. He knows he is. But he still does it. He is now, being seen by the school Counselor, to learn about appropriateness and impulse control etc.
But this kid, he has big cute puppy dog eyes, and even when he is causing trouble and knows it, he will play innocent in it all. And then give his "poor me" puppy dog look.
He even got my son in trouble once. My son knows this boy is full of trouble, so he tries not to sit by him etc. But, the boy will follow him around and sit by him, regardless if my son speaks up or not and tells the Teacher. So once, the boy was bothering my son, my son told him to stop and he wouldn't. So then, my son got so irked and because the boy was not respecting or listening to being told to stop by my son, so my son (who is 6) LOUDLY said "STOP IT!" and he hence, got in trouble for "yelling" in class. And the other boy, played all innocent saying he wasn't doing anything. But so my son, had to explain to the Teacher etc. and it was so, frustrating for him.
So the degree of trouble a child causes in class, can vary, and in scope and in how it affects all of the other students. And sure, if a certain kid always causes trouble, other kids may not like him or her.
Through repetition and repeated behavior, even if no one actually says he is a trouble maker... people by mere observation, will associate a child with certain labels. Good or not.
Many great minds, can sabotage themselves because of their attitude or behavior or bad habits. Smartness, does not equal wise.
And some people/kids, just because they are always told how smart they are... they start to think that they can do whatever they want. Because they are always told how smart they are. And they feel entitled.
Doesn't your son's school, have Counselors???
That is what they are there, for.
To facilitate help for a child. Especially when the problem has become to much for a classroom and Teacher to handle one on one everyday all day.
It doesn't matter if the Teachers are young or old.
Nor does age of the Teacher, determine how capable a teacher is or not.
My son's Kindergarten Teacher, was the youngest Teacher on campus. BUT... she is SOOOOOOOOOO good at teaching AND at handling the kids. And her class, even if it was full of boys...was one of the most behaved classes on campus.