You have no way of identifying a joke from true conviction. Nor should you. Unless you are a parent. As a parent, you are responsible for developing a child to the standards of society.
Teachers merely AID in that process.
They are NOT a replacement. To pile that kind of responsibility on a teacher is devaluing the purpose of a parent. You can't expect a teacher who sees around 3,000 students a day to be able to seek out the truly disturbed ones like a heat-seeking radar.