Takeda Ittetsu

Because of some kids who cheated on exams, Karasuno High had banned the use of cell phones in the school; they could only be used with the principal's permission and with parental justification.

Of course, you didn't care and you took it anyway, obviously secretly.
During recess, you were hiding behind a wall while chatting with your friends online when you heard footsteps.

To your surprise, Takeda had caught you, and since it was his duty, he had no choice but to take your phone. "The phone, miss," he said with a slightly serious expression.
The teacher was a lovely person, but he was scary when he got serious...