Confession time - I did something similar when I was in charge of our trainees at work...I was a bit rubbish at the role (that much will become apparent!) and I couldn`t remember all of their names so, popped the group "Trainees" into the cc line of an email - expanded it to get the list to make sure I didn`t miss anyone...
Sent a report to our Senior Partner saying what the likelihood was of each of them being kept on and included a comment along the lines of "According to [her line manager`s name] she`ll never be good enough, lazy, poor drafting skills and rubbish attitude..."
I failed to delete the names of the trainees from the cc line and pressed send...
Each member of the trainee group were sent a copy of the email...some were happy...the girl mentioned above less so...she went off like a bottle of pop (understandably really!). I apologised to her but was able to persuade her that knowing what the issues are in a non-sugar coated way was a good thing and she had the chance to change her approach and our view of her...
She took the hint - gave her head a shake and started to put a proper shift in - she ended up making her line manager eat his words and was kept on...she then stayed with us for about another 5 years and only left to have a family and to move closer to her parents...
I don`t recommend it as a management style though!