The target of a bully doesn’t need a hero. They need one person who stands next to them in the hallway.

Rescue is not usually what is missing. What is missing is company, because the worst part of being picked on is the conclusion you start drawing about yourself when nobody will come near you. Presence contradicts that conclusion without anyone having to make a scene. It also changes the arithmetic for whoever is doing it, since one person on the other side is enough to spoil the performance.

Walk with them between classes, sit down at their table, say hello in front of other people and mean it. None of that requires a confrontation, and it is worth more than a speech you never make.