This teacher shows he/she has a real problem controlling her emotions.
To put such comments in writing is either poor emotional control or extremely poor judgment.
Either way he/she should not be in the classroom.
I don't think they should mix the slow kids with the fast ones. It is very difficult to teach a class, and be of any use to the high achievers, when the process is bogged down with slower students. Teachers need to be specially trained in order to handle the underachievers; and, their job would be much easier to do if they didn't have any overachievers in the class.
Doing away with fast/slow "tracking" of students is one of the primary reasons that our educational system is broken.