Kubota told him to "Stop". The tool took that as encouragement. It meant his needling was getting to Kubota. The names became more vile.
Then Kubota shoved the miscreant.
The school frowned upon that response. They are sorting through the sequence of events. The idiot was loud enough that there were many, many witnesses. Heck, the whole point of the show, from the idiot's perspective, was "Look at how brave I am, poking the bear in the cage."
Kubota will have consequences. He did not follow the official school policy, "See something, say something."
I am proud that Kubota showed restraint
I believe he could have turned the tool into a greasy spot on the pavement. A "push" is a pretty gentle escalation.
I think Kubota is aware that following official policy just makes bullies sneaky. Official policy requires the commitment of resources to investigate. Official policy bears the burden of finding solid evidence. It is an unwieldy tool to address every abrasion and friction that young people experience in their coming of age.
A push sends a message. It tells the aggressor that he cannot hide behind technicalities and procedures. A push tells the aggressor "I am holding you accountable." It tells the aggressor that the victim is willing to face censure if that is the price of justice.
Bullies depend on victim's silence. Bullies depend on the collusion of shame, both in their victims and the witnesses.
That push shattered the curtain of silence.
Kubota will have consequences. Wisdom entails know when an action is worth the consequences it will trigger.