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 lord of the second house hath the same strength in hurting, as the lord of the eighth; the lord of the sixth, the same with the lord of the twelfth. - William Lilly (1602.-1681.)