Unfortunately, for some children the school is the only secure, stable & supportive framework a child has. Not every child is with parents, like you, who care about them & teach their children good behaviours & the harsh consequences of poor behaviour (like carrying a knife).
For some children, knife carrying & using threatening behaviour is normalised within their family.
I suspect the head will realise there may be little support from this child’s parents. Therefore, the only option to give that child the right support & guidance (and, to be honest, a Police bollocking) will be to have kept that child in the classroom.
Unfortunately too, there are some parents for whom seeing their child suspended or excluded will be a badge of honour, and will not give the child the right moral lessons in right from wrong. If it were my child, their ears would be ringing from the total bollocking they would have received from me, as well as showing examples of how devastating knife crime is, as well as discussing why they felt the need to carry a knife & threaten other children. This kid may not have that kind of moral & supportive parenting at home.
Most parents on here see life within a bubble of setting a good, supportive environment for their children. I think we forget sometimes that all parents are not created equal. For some, carrying a knife & using threatening language is a normal thing. If that’s the case with this child, suspending them will only send that child back into that poor moral environment.
Heads have a tough job balancing the needs of safeguarding all children. In this boy’s case, safeguarding the child may be keeping the child in school away from that crap (and frankly dangerous) parenting. It may be they are aware that being suspended or expelled could put the child at risk from violence in the home.
I am in no way defending this little turd for carrying a knife & threatening your son & his friend. Personally, I’d think that kid is a little scroat. But professionally in this case, there may be factors in this child’s home life that make suspension a very bad thing, and as much as I’d wish every parent would be as good as you are, the school may be the only positive parenting that child gets. Hence why they punished the child by talking away their playtime.
For good parents, it seems like this kid is being rewarded. In reality, keeping the kid in school may be due to safeguarding the child from a home where such threats and carrying weapons is normal.