Do not be intimidated or fall into the trap of losing your temper. As you remain firm and calm, the parent will eventually cool down and meet your level. If it comes to the point where you feel you cannot retain your composure, say nothing. The interview will soon terminate. Please remember this type of interview is a rare occurrence.
Most parents are reasonable and merely concerned about their child's grades.
A letter of concern or the odd phone call can go a long way in eliminating long interviews. Encourage the parents to come to the parent-teacher day right on your comment form. Tell the parent to call you two to three weeks after the interview, to see if there has been a change regarding his child. You have a hundred and twenty students to keep track of. He knows your name and the school's number.