I don't understand this situation, Does anyone know?

Today this Small Mean 7th Grade Kid came up to this big 7th Grader and called him Bisexual and the F word.And the kid in return punched the mean little jerk in the face.Then the mean little kid went crying to the Gym Teacher saying the Big Kid hit him on purpose. But all the Big Guy was doing was defending himself.Then the Gym Teacher sent them both to ISS.But It's not right that the Big Kid had to go to ISS when all he was doing was standing up to a Bully.Believe me I would've done the same thing.So why did they get The Big Guy in trouble when all he was doing was defending himself?

You know that he was defending himself but the gym teacher does not know this.