We know it for sure by word of God (EY) in his author's notes, which he also posted here (sorry, I don't know how to do links: [link]
Maybe he has some ulterior motive for wanting Harry to be supported. Maybe he just wants the magical world to be ruled by somebody competent and doesn't care who does it. It's kind of sad to think of Quirrel as Voldemort because he is such an awesome mentor. I probably won't figure it out until the grand moment of revelation.
Does anybody want to work on a list of everything that looks like a hint? I've already started, but I'm not really good at subtleties, so I'm probably missing really important things.
Also, dinosaurusgede, this is a really cool picture. I like how you can tell who most of the students watching are. Also, it's awesome how you draw Quirrel to look exactly like that statue EY said he looked like. This picture really captures the feeling of that part of Ch19. Keep up the amazing work!