It's supposed to be around 4pm - a couple of years ago someone pressed the button early and we got ours around 3pm - the time before that we got ours at about 4:20pm (I think my get new mail button was worn out by then
It seems to take a long time from them sending the first to them sending the last (I assume they have some cranky old mail system that can't cope with sending more than about 10 at a time). It seems to take over an hour to get from the first email to the last.
In terms of who gets sent the mail first, it's not alphabetical or by school, my best guess from my previous experiences is they are sent out in the order that people registered online.
Good luck to everyone waiting this year!