In the novel Cat Among the Pigeons, Mary is one of the pupils at Meadowbank. In one of his reports to his boss, Colonel Pikeaway, Adam Goodman reported that he had made friends with "three gigglers", Pamela, Lois and Mary. She might be Mary Vyse, Mary King or a separate character, because it's unlikely that she's Mary Vyse as she's described as boring and serious.