Crisc: I am so sorry to hear about this. We have had aggression problems with our DS4 also, and from an emotional standpoint they are incredibly hard to deal with. Perhaps our story will help you think about your situation.
Our problems began last year when DS (then 3) underwent a triple-whammy: new house in a new town, new school (in immersion French, a language he didn't speak), and DW undergoing a severe illness and consequent surgery. The whole thing was compounded, though, by the fact that the teacher allowed the problem to go on for almost two months without saying a word to us. We went to the parent-teacher meeting in Nov., shortly after a serious and unexpected surgery for DW, thinking that at least we'd get some good news about our son. Instead, the teacher went on a tirade about how DS had become the class bully and was regularly beating up all his classmates. It was so unexpected, and so hostile, and we were in such an emotionally vulnerable state anyhow because of DW's illness, that we both just went home and cried.
Our strategy was to demand daily updates from the teacher about DS's behavior at school, and to emphasize social and behavioral issues at home. I expressed my strong dissatisfaction about being kept out of the loop, and told her that we couldn't expect to deal with a problem like this unless the teacher and the parents presented a united front. (Some of the issues here might be cultural: the French teachers are much more reticent to talk about your kid than American teachers seem to be.) Since then our mantra every morning when we drop him off at school is that he has to play nicely with his friends and listen to his teachers. Sometimes that works.
At the suggestion of the teacher, too, we enrolled him in karate. This has had the expected side-effect that when there are issues now he is capable of doing more serious damage. (Indeed, my immediate response to the teacher's suggestion was, "What? You think he's not efficient enough when he beats up his friends?") But it is true that given the right instructor one of the main lessons of the practice is self-control. In the end, I think that karate has been a valuable thing for DS. But the main advantage of it is that he reveres his karate instructor. (And so do we, by the way.) One very good way to get him to think more carefully about his behavior is to ask him whether Mr. D. would approve. In general, I think it is terrific for him to have another very powerful male role model, especially one who endorses physical behavior but teaches the boundaries. It's also a pretty good physical outlet.
Like you, I'm not at all sure that it's right to put our problems down to boredom. Indeed, for the most part he seems to like school. Last year I think the triple-whammy was the explanation, but this year it seems to be a bit different. After doing 180 degree turnaround last year, the problems recurred again this September. In this case I think he really was being incited by one of the other boys. Fortunately, that's the way the teacher sees it too. But I did try very hard at the beginning of the year to establish an open line of communication with the teacher, and we have been able to present ourselves as working with her. I think that has helped a lot.
Frankly, it also helped a lot last year to take a very hard line. I found the things I thought DS most enjoyed and threatened to take them away if the behavior continued. And then I followed through when it did. I think that established a certain kind of relationship between us at the age of 3 that was helpful in dealing with behavior problems. I don't know if it would work the same way at 6.
In any case, I hope that some of these stories give you ideas. Feel free to PM me if you think it would be useful to have more details. And best luck...
BB