Hmm, another good point. Maybe picking the girls' favorite classes will increase their mood, or just participating in the lessons. Or maybe a night without work will also raise it up to a certain level at least.
And in the current build, there's only two additional classes available, I think. Bartender and Bar master. Bartender is unlocked when a girl's drink mixing is at least 25, their conversation at least 20, and their hostess expertise 75 or below (it's a weird reverse stat - 100 means she has no experience and by 0, she's a veteran at the hostess job).