Yamaku staff subtly, but purposely, pair students together to form an innate Buddy System.
Ideally, teachers try and pair a student's buddy as the person they sit next to in Homeroom, but they wont stop this system working across different classes so long as they actually find a pairing (Mutou tried to encourage Hanako to befriend Akio, but over time he saw she felt more comfortable with Lilly and settled for that).
On a slight tangent, my headcannon is that Mutou is a childless husband and can find himself being quite protective of 3-3's students, becoming somewhat paternal towards them. He's known to cover for some of his student's bad behaviour and actively reminds 'his kids' that he always has an open door policy, despite noone ever taking him up on it.
