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I think you need to admit what you did, and say what you learned from your mistake. Don't try to hide it. Don't give an excuse saying that what you did wasn't a big deal. Just say that you did something immature, you learned ______ from your mistake, and since that incident you have become a more responsible person.
I don't think it will hurt your application if you're honest about it. On the other hand, if you lie about it and adcoms find out, they will reject you.
We all do some sort of unprofessional conduct. Some worse than others. It just depends on whether or not you're caught.