Uggggg! I can feel the anger rising in my belly as I read you story! How dare she! Seriously how fucking dare she! What you're describing goes way beyond discouraging a child, what THAT teacher did was ABUSE, plain and simple mental and emotional abuse! She could have left it alone and mentioned it to you discreetly, but she chose to force a 6 year old child to erase his stuff and then exculded him from the class until he calmed down. The only reason to do that was to deliberately bring him down a peg or two. What kind of a person feels the need to "bring down" a 6 year old? She should have been fired and never let anywhere near a child again as far as I'm concerned.
I'm so sorry that happened to your son. You're absolutely right, that stuff can stay with a kid for years. I hope he's doing okay now.