Make sure the comment is sincere. Don't lie, but surely there is at least one positive comment you can make. Apply this idea to your written comments on a report card. Parents arrive with the student's report card so they can keep track of teacher's names. Ask to see the report card at the very beginning of the interview .
Some parents play a game of intimidation.
They will insinuate that you were the only teacher who gave Johnny a low mark. The truth is you will be on par with the other teachers. The proof from the marks and other teachers' comments will change the tone of the interview. Honesty will come into play. Keep the parent on topic.