A classroom in a dream tends to bring back the specific pressure of being evaluated -- called on, tested, or measured against a standard you didn't set. It often surfaces during periods of real-world scrutiny, translating a current performance worry into the most familiar setting your mind has for that feeling.
The swift and various motions of the Planets are to be considered, as also their slowness, because when they vary, the accidents of the Aspects do alter. - William Lilly (1602.-1681.)