My eldest DS is 10 very nearly 11 and he was like this, and I think it was the very start of a gentle curve into pubety. Sitting still in class and complying with school rules and standards can be very frustrating for them, they work so hard at it all school day long! We got DS a punch bag, one that is on a stand, that he can just whack when he wants and it works!
As women we don't tend to understand maleness, to us it can be seen as a violent outburst that is not acceptable but to a boy/man it is a way of release one's feelings and frustrations and no it does not lead to be a violent person in 99% of cases. We may sob and whine and be moody! I think trying to analyse it and ask him to think about what he could do differently is no help, for boys the anger and frustration is out and that's it! Of course he can't react to his classmates that way, but he sounds like he needs another outlet for the fizzing he feels, my DS loves to boot the hell out of a football round the garden too. He can come home from school foul as hell, but after a few boots of a ball, food expecially some protein he is a different boy.
There is always some unfairness and stupidity in game playing like you mention, he just needs to learn to deal with it, it used to make my DS crazy too, but now he shrugs it off and gets on with the game, plus the less fuss he made the less others did it to him and each other.
Y4, 5 and now 6 get harder and harder with more pressure and expectation, so they are trapped in a classroom for hours and then need to go off like a pressure cooker. My DS is also dyslexic and he works so much harder to get the results he gets and it takes its toll. Just like a really shit & hard day at the office sometimes! Maybe find him an outlet he enjoys, get the physical side working with the mental side, football, gymnastics, judo etc. Is he in Cubs or Scouts, this is the perfect balance of discipline and fun.
I also let hom know that anger being released is a good thing, all emotions are, but like all emotions there has to be some management of them in certain situations. Once i gace him a rolling pin when I could see he was so pissed off about something at school and he battered some sofa cushions and then gave me a massive hug and said he felt much better!