This is not a straightforward situation. If you were the parents of children in the class who were threatened by this child, how would you react? What would you have wanted the staff to do? Would you have wanted this child removed from the classroom and your children kept safe? I think I would have.
In terms of the room being a cupboard, it is undoubtedly a DMism, ie an over-exaggeration of the facts and unlikely to be true.
Was there a knife involved? The Telegraph seem pretty sure there was. How would you react as a human being if confronted by a child with a knife? Even with training, this would be a difficult situation to handle. It's hard to know the severity of the 'offence' that caused this situation and the severity of the consequences doled out by the staff.
In terms of children with anger issues and difficult/challenging behaviour needing one to one support and understanding, how would that be funded exactly? We scarcely have enough funding in our school to pay for photocopying paper to produce resources for the classes we teach. I don't think that's gonna happen.
I know that children often have an escalation process with anger issues or behaviour, but they can also be unpredictable and quick to anger, with little time to put intervention strategies into place.
Regardless of what thoughts this triggers, any conclusions we reach are based on pure conjecture and without the facts, I wouldn't like to condemn the staff or the child involved in this.