Job application help.

I would like some insight on what I should start putting on my applications. Here's my situation. Back in June of 2011 I was fired from a daycare becuase I got involved in a situation between two of my colleagues. I know it was stupid. One of the young girls would come in my class in the evening and not help out. She would sit on her phone and not pay attention to the kids. The director knew it but she didn't do much about it. It would frustrate me and another coworkers. Well one day the girl was helping out in the classroom next to mine and she was on her phone and a child ended up walking out of the class. The child was only 18 months. The other teacher told me about it on playground and we both confronted the girl because we were tired of her not doing anything and that child could have gotten hurt. I know it wasn't my place to say anything to her. I was only 20 at the time. Well the girl went and told the director that we made her feel uncomfortable so the director fired us both. I know I shouldn't have said anything to her and I felt stupid. I've learned my lesson. So my question is what should I put on job applications? There's always that question that asks "have you gotten fired before any why?" What should I put? I've been putting "difference of opinion" but it never seems to be enough so I've been scrambling to say something else. Luckily it's never stopped me from getting other jobs after that but I want to put something more professional. Any ideas on what I can say? It was almost six years ago and I find very frustrating to explain this over one dumb mistake.