"Why?" You ask.
"To make him stop," says the child.
Hold on, back up here. The key words are right there: "To make him stop."
This means the child may have tried to walk away or block the blows before throwing one to stop the assault. Don't punish your child for that. Yes, it's bad to fight. It's wrong to hurt people. But we all have the right to defend our personal safety when it's threatened.
A child who is defending him/herself didn't start the fight. The child fought back to stop the fight. Now, had the child kept hitting after the fight starter went down, THAT is an issue where it becomes wrong. But a swing to knock down a bully to stop an assault? I don't consider that a bad thing. In fact, I fully advocate signing a child up for martial arts so they can protect themselves in a safe and controlled way in the event they're assaulted by a bully.
Here's the thing: a school just sees a fight. I understand why they have to discipline all children involved. However, I don't think the child defending him/herself should be punished at home for it unless they went far beyond defense and started fighting offensively. Punishing a child who swung once in self defense only teaches them that they have to lay down and let a bully beat them up, and that is NOT right. Especially not in this day and age where kids sneak weapons into school.