Is ODD a mental disorder or just an excuse for bad behavior?

Here are two stories – both with the same root cause.

In one, the family of a high school senior was suing the district and the school over a tweet the student sent out defaming the principal - among other disciplinary matters.
One of the reasons given was that the child suffers from ODD – or Oppositional Defiant Disorder – which according to the South Jersey Times, quoting the lawsuit, is a condition "characterized by oppositional behavior and difficulty with authority."

Long story short: the district settled with the student’s family and will amend it’s social media policy.

In the other, the Manalapan Police Department is being sued for using force on a black special-needs child. According to the Asbury Park Press:

So far the district is on the hook for $33,000 in legal fees.

Much like I documented the struggles of Robin Williams - his depression – and the stigma attached to it; is it then fair to ask whether or not ODD is truly a mental disorder – or just an excuse on the part of a child to “act out? Have you ever known anyone who's been treated for ODD?