Why should the teacher be subjected to mandatory drug testing?

As a teacher, this issue has come up more than once.  The main point in favor of teacher drug testing is that, since they work with young people and are paid by the public/government, they must be held to a higher moral standard than ordinary citizens, so much so that they should be drug tested regardless of whether or not there is any justifiable suspicion of drug use.

The idea that they are supposed to "set an example" of law abiding conduct, or to uphold the accepted morals of the community in which they live and teach is another argument.  This is along the same lines as teachers being fired for getting a DUI.  While the illegal actions happen outside of the school day, most people tend to believe higher standards of behavior for teachers should be expected and enforced.