So, in the first example:
Who grows up? The teacher or the student?
Boa tarde Osias!
Just like Dormousse559 said, it's the teachers that have to "grow up".
I just want to add that it's an expression mainly used by younger people in France.
It's kind of a way to make fun of someone, or a category of people. Or you can use it when you are exasperated by someone or something.
Like in this situation, obviously the student knows that teachers are mature people (they are grown-ups), so he is not really asking them to become more mature. It's just a funny or mocking way to show that he doesn't agree with his teacher giving him 9.9/10 instead of 10. He thinks that's stupid.
Espero que você entenda!