I've long thought that a B- paper is often the hardest kind of paper to grade, as it is not so obviously bad as a C (or worse) paper; at the same time, it is often hard for students to see why thier paper isn't an A- paper if there isn't anyting fundamentally incorrect in it.
Now there is another reason to worry about B- students.
That threat is both wretched and hilarious, simply b/c of how poor the student's grammar is. Honestly, the fact that he had no clue how to end a sentence or use a comma leads me to believe a B- was probably far too generous a grade.