He was slapped first and then punched the student that slapped him. The school is a psychiatric therapy in nature and not a regular public school. They left this off the title.
I get that when "students were kicking the staff members while they were 'trying to control one of the students,'" tensions get high pretty quick.
However, one student slapping an instructor, at least somewhat separate from the other conflict, does not warrant the instructor punching that student in the face. "Punching in the face" is not a "restraint technique," and from the contents of this short article, there is no indication that the instructor was in fear of their safety or the safety of others based on that slap.