Quite a few reasons, really. I think you might be giving a bit too much credit when you say that they can't know what's going on.
- With the number of students witnessing what was going on before Arata even arrived, word would have spread. There's no way wind of it would not have reached any even if a teacher wasn't explicitly called for at that moment. Especially when you look at the reactions of the students witnessing it. Postures and reactions of nervousness and distress.
- Any nearby teachers, or general staff that could be walking nearby making their way to class, the staff room, storage, whatever. Or even if there was one present in any classrooms next to the ordeal playing out
- Why wouldn't a teacher investigate someone screaming "WHAT ARE YOU GETTING SO HEATED FOR?" "IT'S JUST A JOKE!" "WHAT THE HELL IS WITH YOU SIBLINGS ANYWAY?", so on and so forth as what's being said gets progressively more and more aggro and personal. Especially when she starts screaming "FUCKING DIE!!" and repeatedly screaming die.
- There's when it begins to get physical. If students watching this go down didn't do anything before, surely they would have once that girl started getting physically violent.
- Even if teachers, or any nearby staff, were called for when she began getting physically violent, there's still none to be seen as she keeps hitting and hitting to the point where blood is drawn
The point is, plainly and simply, it's stupid that Arata not only got there first but that there's no staff to be seen before and after it escalates. Like, obviously it's manga so there has to be a reasonable level of suspension of disbelief given here, right? I just find it difficult to give any for this when it's just
that unreasonable to believe otherwise.