My middle child has had a problem with tantrums from a pretty early age. Even when he was at a normal age for tantrums, his could be pretty epic. Like, storming around the house knocking over chairs and then doing this weird "I'm not leaving my bedroom after you have sent me to it." It can go on for hours.
Although he no longer tips over furniture as much, he can still get into a strop that lasts for hours. The problem is now that he will do this at school. He may spend an entire day crouching in the cloak room or somewhere refusing to participate or even come out of the classroom at the end of the day. This can start because something has upset him on the school run or if he's told off by a teacher in class. He gets really upset and can't get out of it and then will act up and compound the problem.
A few years ago I was cuddling him during one of these and I said, "baby, you need to calm down" and he said, "I can't!" I think he gets very angry and then doesn't know how to calm down - and even gets angry at himself for having been so naughty in the first place. Like, he is upset then he's told off and then he runs into a corner which exasperates parents/teachers/whoever and so he's now even more upset and he just digs a hole and can't find a way to walk away from it. I think he wants to be able to calm down and doesn't know how.
I do know that I have been able to jolly him out of some of his strops. At home, we sometimes have success with leaving him to it for a while and then perhaps he can be enticed out of his mood/corner with a bright cheerful thing to do or eat or something (so long as this temptation does not appear to be in any way related to his strop.) Obviously, at school, he can't be handled this way. He needs to be able to shake off what has upset him and move on.
How can we help him learn to do this?