You should not insult the abilities of others.
Mary is working on a group project with other students, she doesn't like some of the work one of the other students has done.
Mary wants to turn in the group project and get an A.
Mary talks with the student about their work and makes some suggestions to improve it.
Mary tells one of the students they are doing a bad job and to let her do it.
The student accepts her suggestions, and when they turn in the project everybody get's an A.
The student tells the teacher and Mary gets given an automatic C.