Nice start. I couldn't help but notice Dumbledore's duplicity. It's all fine and good if Dumbles kills Harry to keep him from becoming the next Dark Lord, but what does that make Dumbles? A Dark Lord. I wonder how many compulsion charms he cast on Moody, Dung, and Tonks to convince them to attack Harry? Dumbles has gone round the twist.
I have only one complaint and that is the size of the "small room" you described in which Harry wiped out Tom's cannon fodder. 100 yards by 200 yards is twice the size of the NFL regulation playing field. That's not a small room, even with 150 people in it. Now...10 yards by 20 yards would be cramped for that many people.
Anyway, good job so far, so I'm going to read on.