Keep doing other quests (specifically Rachel's or Priti's, I think). I don't remember exactly what triggers it, but eventually the nurse will come to you on her own at the start of a weekday and will then teach a sex ed lesson the next time you teach a class. After that, you'll be able to advance Lucy's quest.
You also have to complete Priti's quest before you can advance that one anyway if I'm remembering right.