I think you and your son need to go with your gut on placement. While I wouldn't ignore the teacher, her critiques do not sound very constructive or professional. Her job is not just to find the students' errors, but to nurture their young minds...what is she doing to HELP them develop?
It's hard for kids to find that happy medium between working efficiently and carefully. DD has a "quick brain," so I am CONSTANTLY reminding her to double-check her work on tests. She tends to make MORE errors in math when she finds something easy. When she double-checks, she gets amazing results. I think that it is great that he has moved away from perfectionism...hopefully, this will allow him to not be afraid to try challenging material in the future. Perfectionism really does have a downside.
Yes, vent away! We understand!
I would find this frustrating, too.
