OVerall message: He is saying that he is fine as it is.
If I understood properly, he says that people normally give a part of themselves to others in order to connect with them. The kid does the opposite and doesn't gives a lot of himself to others, and it has never really been a problem through his life: the grades represent that he is smart but keeps to himself, being good at sports shows that he is not doing it because he can't cooperate or because he feels shy about showing himself, having a good friend circle means he is not a recluse that doesn't knows how to interact with others, the fact his parents are just average shows that he didn't become like this out of trauma, deppression or anything as far as meets the eye. He just has his way of going around, and that gets people worried about his future even though to him he is just doing alright.
Maybe he is shy, maybe he really doesn't likes to be with people, maybe there's more to him and he actually has some problems not concerned with school. It's hard to say because he doesn't gives a lot of himself to others as the teach said, if a normal person connects with others through 75% out of 100%, this guy connects , communicates and works along at just around 50%. The 25% seems to be a joke on how when the teacher got surprised and made a comment on his way of giving himself to others, the student didn't keep speaking even though he just corrected him, so he may actually just give even less than half a quarter because he just gives "enough".
at least that's how I took it.