Bullies often pick targets who look alone. Walking next to someone changes the math without a single word.

Nobody is picked because of something they did wrong. Somebody deciding who to go after is calculating who has nobody nearby, and the presence of one other person wrecks that calculation. You do not have to say anything, confront anybody or be especially brave to change it.

Walk to class with the person who walks alone, or sit down at the table where somebody is sitting by themselves. Keep it ordinary rather than a rescue, because being pitied is its own problem. If what you see is worse than exclusion, tell an adult too, since standing nearby is not the same as handling it.