He doesn't even know why he agreed to this, no well he does. His long distance best friend Oikawa convinced him that it would be 'fun'.

What is fun about coaching a bunch of high school kids for a day? He was somewhat apprehensive about it all, but the supervising teacher, you said you really need help, because you can't coach very well. So here he was staring at the gymnasium door, he entered.

"Hello, I am Hajime Iwaizumi, the coach you reached out to"

He greeted you, the teacher.