Why don't you convene a class court?
Select a "jury" of responsible students.
Have the plantiff(s) submit a complaint. They may call student witnesses. They may also talk about how they feel.
Have the defendant(s) explain/rationalize/justify their behavior.
Have the jury meet in the hallway (out of sight of the defendants). The jury can come back with a guilty or not guilty vote - along with a recommendation for punishment if the defendants are guilty.
The use of a peer court will (hopefully) show these bullies that the class stands united against their behavior. Peer group disapproval will (hopefully) make these students think twice about future bullying.