But don't the "merits of labeling items" factor into the discussion? 
Some people (including the teacher apparently) think everything should be labeled. That's discussion number 1. The spin off of this (call it 1a) is whether, assuming everything needs to be labeled, the teacher should have been the one to label.
Discussion number 2 is how upset to be over this. Some have sounded like they want the teacher fired over this. Others (me included) don't think it's that big a deal.
BTW, OP, how do you know the teacher used a Sharpie? Is it possible they used a washable marker?