Eh... Notice how he just seems to vanish when other people show up? Not 100% sure his even real.
I was thinking that too but
Someone must have told him about Asakura being after Sawashiro
Someone must have fake called the police to scare the bullies
Someone must have filmed the bullies to get them expelled
Someone must have taken him to the hospital
Those are things that are happening outside of Yamagata's head, now the last 3 could be from a complete strangers really just passing by and deciding to take action, but the first one... actually if he has super schizophrenia the first one could be in his head, he just filters the rumors and then re arranges them to be delivered to him by a second personality
But that would be too crazy, too... mad for a pre alien invasion world