How do you unlock Draco Malfoy in Lego Harry Potter years 5-7?

You can't unlock Draco Malfoy in Lego Harry Potter Years 5-7. He's a playable character from the start, available for selection at the beginning of the game.

He's not unlocked through a specific mission or collectible, he's simply a part of the roster of playable characters. You can use him from the very beginning of the game.