I may have missed it, but I do not recall that 6rh year students were taught to make Veritaserum. Slughorn said "These are the kind of thing you ought to be able to make after completing your N.E.W.T.s.", but he never said that they would be making it during a lesson. As an analogy, a chemistry major would have the skill to make synthetic narcotics, but that does not mean it would ever be part of a laboratory assignment, for obvious reasons. Given the fact that Veritaserum is strictly regulated, it is unlikely that this would ever be part of a lesson due to the danger of a student covertly obtaining some - as Malfoy in fact did with the Polyjuice potion.