Get a back issue of the April issue of Lapidary Journal (every
year). It has an extensive listing of schools all over the US.
If you have a specific type of class you are looking for, or can
be more specific in your goals (hobbyist, professional training,
geographic restraints, etc.) you could post the list and we can
give you recommendations.
Also, if you join SNAG, they might have/sell a list. While you
wait for your Lap Journal back issue to arrive, go to the
bookstore and look at the ads in all the metal magazines.