I think I had this happen once. As I recall, some other classroom event got priority over Emily telling Rachel about the DLC, but the story advanced anyway. If you don't remember getting that scene, teach a few classes and see if shows up, then spy on her that night.
Had that sort of happen in Chapter 1 too: when teaching an afternoon class, if I already had a girl scheduled for punishment, and I get the Cassandra scene where there is no public option, it will replace that girl, but it won't respawn the event for the other girl right away (at least for Amy and Debbie, not sure of the others) it instead takes several days, maybe a week?