This means he was always using willpower to be nice to other children. Call it acting. He was good at it, so good people wouldn't realise he was acting for week

This means he was always using willpower to be nice to other children. Call it acting. He was good at it, so good people wouldn't realise he was acting for weeks or months. He was not "suddenly violent", he was pestered for days/weeks/maybe even months and eventually lost control. Nobody turns suddenly violent without reason, you've just missed something. This long list of one-sided remarks with no explanation is typical of schools where teacher supervision is totally lacking. Of course they still see the explosion, no matter how bad the supervision. What they miss is the reason. They see how it ends, not the buildup towards it. And, of course, the buildup is the problem, not that a kid eventually loses control. Every kid and adult will eventually lose control, given enough stress. That the kid was able to do so much damage is also an indication that oversight totally failed. Was there no warning sign before that helmet shot? really? Yes, of course there were. Many of them. They just totally missed everything. It isn't that nothing was happening in the class, it's just that shooting another kid in the face is the minimum threshold for getting to a teacher's attention that maybe some supervision is necessary. Then, of course, the teacher demands some extreme punishment and declares "everything went so fast". Everything went "so fast" while this teacher was looking at these kids for hours ... Either their ability to predict social interactions is worse than a dog's (unlikely), or the teacher was playing angry birds for hours at that point. The second factor also explains why the teacher demands extreme punishment: to hide that they're not supervising the kids.