User blog comment:Maki Kuronami/Lesson 18: Ruby, why you so bad with faces?/@comment-3257939-20140801021006/@comment-2006510-20140801022444

As far as remembering, we must also remember their battle began before Beacon's first semester - the latest episode began at the start of the second ;)

So unless semesters are different there, that's six months between meetings. Ruby could have easily forgotten, but Glynda migth be suspicious - either way Cinder will have to be careful about her Semblance, because that'll give her away immediately