I do think it's not random that none of the students died, though-- and that's a big part of why I always thought Draco wouldn't, too. It's mostly to do with generations and renewal-- if there's one thing that's predictable in her plotting, it's how JKR functions through generations like tiers, with each one starting the same party over again (even, or especially, to the point of Albus Severus...). The Marauders were over, so they died. I forgot about Colin-- he was a student. So I guess it really is that the people 'with Harry', on his tier, have to rebuild the same dynamic, basically. He wouldn't lose any of the Trio and it seemed unlikely he'd lose anyone who could be counted as part of his... story, or generation, or what have you. Or at least, I see reason to the madness :>