>>536968154
Maybe the secret boss will be Papyrus and you get his Crystal in the middle of the chapter in a forced encounter. Unlike how in chapters 1, 2, 3 where you get the encounter at the final dungeon. And unlike Gerson in which you can only fight him in the first dark world but not in the second dark world.
That way, you could head to the school after beating the first dark world, get your pure crystal, then face off Asgore.