As a longtime bartender in this city... if you're interested in working behind the bar, don't go to bartending school. It could be useful for picking up some basics if you have very little knowledge to begin with, but don't put it on your resume or bring it up in the interview. Instead, start as a server or barback and let management know that you're interested in working your way up. Much more practical experience, real people experience, and managers will take you seriously.

I'm not sure what the age requirement is for different places (18+ or 21+), but consider working as a bouncer or as a security guard at bars, nightclubs, or music venues.

I think it's likely you're looking for LBJ Restaurant (500 First St. NW), which is referred to as a jazz club in this WaPo article from 1980. This old DC directory also has LBJ's listed under "Bars and Clubs."

If correct, the club's location is now an office building owned by Georgetown University.

(Edit - links)

Planta!! There’s a location in Bethesda and they’re opening a second one in Foggy Bottom soon. Lots of options including a “spicy tuna” and a “baked crab” roll.