For anyone who needed to gauge just how far the demonization of teachers unions has gone, the final episode last spring of TV's "Law and Order" -- the real one, not the new L.A. knock-off -- was instructive. Bracketed by the customary soundtrack "cha-chungs," the story concerned a loony former substitute teacher bent on blowing up the high school where he'd subbed. But the real heavy was a weaselly union lawyer who blocked the cops' access to the teacher's whereabouts, on the grounds of -- well, something like Marx's labor theory of value, or Walter Reuther's argument for co-determination at General Motors; it wasn't entirely clear which. Said weaselly lawyer crumbled, of course,