I agree that the teacher doesn't sound like she's on top of this, she's taking a bit of a heavy hand with a wee little prep. I think your son needs a safe place he can go when distressed, so that the teacher knows where to find him (preferably in the classroom I guess). I think its a good idea to mention the psych & try to postpone a trip to the principal. If the teacher will work with you in isolation, it might make less of a "deal" of it for your son, and talking about the psych's methods might lend some weight to back up your approach (not that you should need backing up, you're his Mum but you know what I mean)
I'm on phone so can't see names but many thanks to the PP who mentioned that list of 10 bad things. Dd1 gets very frustrated at little things to the point where she destroys work & refuses to continue, so I will be doing this with her in the very near future. Its an excellent idea!
ETA it was patsmum, thank you