First I would ask is this a class for a potential employer? If so, then obviously the other ideas already presented on "conservative" office wear, twinsets, jacket, etc. is very appropriate and....critical to your initial perception by the new employer. And remember that in some conservative career fields hose are still required. [It's always easier to "dress down" AFTER they find out how irreplaceable and brainy you are!]
If the class is something along the line of career advancement -- but not through your employer (or possible future employer) then I'd say be neat, clean, pressed, and you're good to go.
If you're not looking to shop for new just because of this class, then use the two black slacks with all your available appropriate tops, mix in a couple of the dresses with jackets, and call it good. It's perfectly fine to repeat outfits.
One last thought..........in the event that you might make some useful contacts during this class I'd say triple check yourself in the mirror each day before you leave home assuring that you can bend over (i.e., like bending from the waist and looking at paper resting on a desk), sit, stand, etc. without showing anything (either upper or lower) that someone else with a very very conservative bent would even remotely find possibly off-putting because that might lose you that person's referral to somewhere you really really want to have an interview with.