Class 3B was a class at Coal Hill School, taught History by Clara Oswald. It was full of boys who did "not wonderfully well, but not awfully", according to Clara. They tended to scrape their subjects but not understand what was being taught.
In the Prison, Clara told Bentley that she treated the Doctor the same way that she treated the boys. This unique approach was what the Doctor referred to as "Taking Class 3B for History", and was requested by him when approaching an armed and angry Bentley. (PROSE: The Blood Cell)