Update (reminded by Spexxvet linking to the discussion)
Kid B was moved to a different class after Christmas, no explanation, life became much better. Kid A has learned to avoid Kid B
Principal also assured us kids A and B would not be in the same class next year.
Principal resigned last week
There have apparently been many complaints about similar poor handling of situations, principal (who was on a year's leave to get a masters degree) has decided to pursue a different career.