@HYBRID_BEING
I think in Japan, there are set of rule to put students at which class they will take. But mostly it depends on the homeroom teacher decision and based on reports about the kids themselves. Maybe most Asian schools are like that.
siblings will never been put in the same class even if they are twins or non-blood related, bullies will be marked and usually put at lowest group, and talented / smart children will be put in Class A (first class) mostly to get special lecture and supervision, because if talented children win tournament, it will be a good achievement portfolio for the school actually.