Have a sit down with your boy/girl and tell them that that bully wants something that he just can't obtain. To keep the bully from persisting your boy/girl needs to find a better school. Or possibly the bully and or bullies could be relocated to a more private school.
This still could cause a problem. If the right form of schooling is in place for the bullies there would be backlash from the innocent because they have nothing special. For your kid to be able to go to school the next day and the next day. Reminder: Bullies are like bulls they see red where there is a trigger in their brain that makes them angry. Tell the kid that he or she must act oblivious to the situation at hand and to ask for some sort of intervention. The obviousness of the problem also could be that there is an underlying action that has not rose to the surface. Ask your son to be upfront with the problem first with you and your spouse. Then the problem will be in the bullies or bully as a solution. Only in school do they have the power to stop bullying while it happens. If the bullies don't like their aggression forced back in ther f ace I am sure that the problem cannot persist with a bully being bullied.