Here's one explanation: http://www.christianitytoday.com/pastors/2010/september-online-only/what-did-jesus-mean-by-judge-not.html  There's a difference between discerning that an action is right or wrong, and stand for right, and deciding the person who did wrong is irredeemable.  We are taught to discern right from wrong, but not to treat someone as garbage, because they committed a wrong act, aka, sin.

Her actions were wrong, being against the school behavior code.  That's discernment. And she is taking responsibility. Shunning her forevermore would be judging. So we have discernment, which is biblical, and discipline, which naturally follows. The discipline is not the judgment that Jesus warned against. The social media harassment is, and should have been addressed strongly by school leaders. Removing her from leadership is appropriate; pretending at graduation that all students are perfect angels, which is what they seem to want to do, is silly.  JMO as a conservative Christian.