Lagging behind is generally used in the sense of not catching up, as in Jessica always lag behind, looking in shopping windows. (Longman Contemporary English.)

   In other words, it is more used to compare to something or somebody else.

   Usually, it is used to talk about the student that can´t catch up with the rest of class; I never had heard it to talk about the teacher up to now.

    Anyway, I let the last word to natives  and teachers around.  Comments welcome, then.