In Episode II, we see this interaction between Obi-Wan and Yoda:
OBI-WAN One more thing. Jango mentioned he was recruited by someone named Darth Tyranus. Any idea who that might be?
YODA With the forename Darth, a Sith he must be.
Did the Jedi not teach about the Sith? It seems odd that Obi-Wan didn't realise that the name Darth was a title pointing to their mortal enemy. I'd prefer a canon answer, and while the Jedi may have believed the Sith extinct, this implies that they already had at least SOME knowledge, from where not knowing about the title Darth would seem to be a peculiar oversight (too much so to be credible).