Even so, I guarantee some will not respond. I'd say after two weeks, you choose the four winners from those who completed their duels.
I think each student character should state their preference. Personally, even if I enter my students, I'd prefer them being paired with adults, even though they will be losing consistently. It's more fun for me. XD Each student's RPer should decide.
The winners would not be chosen one per thread. If the RPers ended up doing that, and finding a winner, I think that's great. But most likely, the duels won't actually 'finish' as that would require someone passing out or giving up. We'd all review all of the duels and pick the four we considered to be the most clever with their spells or whatever. We'd have more than 4 duels going on anyway, but I'm guessing 4 people would be a good number for the wrap up main winner scene.
For example.
Player A and B were duelling. Player A may have had the upperhand the entire time, but B was using a wider variety of spells, was more accurate, and was also clever. B would be picked.
C and D duelled and C was nervous and kept dropping his wand. D would wait each time before pressing the attack. D would win.
This is the sort of things that I hope would win. Some people will be unrealistic with their duels, so I think the characters that are written realistically and with effort would be best deserving of finals.
But I think we should do a few weeks of just duels without main winners first. To get a solid base of people.