Suddenly, Harry remembered the Room of Requirement when it had been used by the DA in seventh year. He remembered what Neville had said - that it opened up doorways into random parts of the castle when the DA wanted to leave without being caught. And then, Harry remembered the permanently empty cupboard in the classroom behind him. The cupboard that had once acted as a doorway for him and Draco to enter their Room from. Harry knew exactly what to do.