Your opinion doesn't mean anything when discussing the law. Exactly what was this kid doing that was such a PITA? Even if he was, you don't physically assault the kid.
And the law applies the same to adults and children, you can't attack kids because they annoy you.
I don't think you understand just how brutally savage some of these kids can be. As I've previously stated I worked in schools for years as a Teacher Assistant. My job was take certain kids out of the classroom and work with them one on one because they were just too much too handle.
To give an extreme example, I once worked with a girl who was only 6 years old and already had a tragic history of sexual abuse. When she would lose it in the classroom she resorted to physically attacking other students and screaming obscenities at the teacher and in front of the students: "You want to fuck me you whore?"
Eventually this girl was placed into a different program for abused children, but it took some time until the school could move her into a more suitable program. During that time the teacher had to somehow manage her on top of the regular classroom duties. The 6-year old students were victims of violence and sexual language.
Of course, not all students are as extreme as the example I gave, but you would be shocked at how nasty some of these students can be. Civility is a learned behavior and unless someone teaches these kids how to act appropriately before they get into the classroom its going to be hell for the teachers and the students. The "kids are precious angels" bullshit mentality has to be changed.
There were classes I worked in where the teacher spent nearly the whole class dealing with a couple of shit-heads for 40 minutes and then barely had time to teach the lesson.
From what I have experienced I really believe that behavior standards need to be set in order to prevent certain behavior problem students from ruining classrooms. I also think parents should be punished in some way when their kids behave like savages.
As I previously stated, every single problem kid I worked with had terrible parents.