Probably because, as you so rightly say, he's a pompous, self important ass. And he's so full of his pomposity that he's not even stopped to consider that there's no punishment that he can mete out these ex-pupils. They're no longer under his command. He can no longer give them lines, detentions, or the cane, should they refuse to do his will. But he's so full of his self-importance that he's unlikely to have stopped to consider that fact. He probably still assumes that they will obey his every command, just as most of them had reluctantly done for the seven years they had been pupils at the school.