You're both so totally and obviously wrong! It's clearly the other way around, because if it weren't, I'd be wrong, and that CANNOT happen! Trust me, it's a scientific fact.
The way I figure it, if there were 20 slots available, and you picked one, you'd be the first and thus in the AM. I picked 10/21 as well. There was one slot left when I registered, so I'm guessing PM, but I wrote the school about it for clarification. I know that appeal to authority is going to totally screw up our debate here, but whatcha gonna do?
Good luck!