Arrest the kids and charge them as accomplices to murder.
That doesn't make sense. Kids in High School today are required to be totally non-violent and disarmed. If a law abiding student brought a defensive weapon to school they would be immediately prosecuted. A kid who tries to stop a fight is likely to be arrested for assault since they probably could retreat from the fight safely.
It would be unreasonable to expect an unarmed, untrained student to attempt to stop an attack by multiple individuals using one or more knives. You need serious martial arts training to successfully defeat multiple adversaries with at least one having and using a knife if you are unarmed.
So in many cases students record what is happening since that at least provides evidence, and they have grown up in a world where taking pictures and posting them on social media are the actions they have been allowed to take. Nobody ever let them fight, or work to physically defeat someone using violence. So they don't even know what to do. But the criminals of course do know what to do.
Another reason people need to be able to arm themselves, since criminals always will.