The application procedure was rather different when my ds, now a DA student, applied a few years back, but he's spoken with plenty of kids who have been through it recently and he says that most of them mention the assessment day more than anything else. From the parents I've spoken with, many of whom are now applying for a sibling of a current student, I get the same vibe. That said, I haven't got a crystal ball and I have no firsthand experience of the current assessment, but for what it's worth....
My understanding is that the assessment is, or is close to, a full day at the school. The kids involved form their own classroom for the day, and they really don't have an opportunity to mingle with the current students. They are taught mostly by one of the middle school English/history teachers, though a math teacher comes in for that portion of instruction/assessment, and the same goes for science whenever science assessment is included. Some kids come out exhausted, other's don't seem particularly fazed -- guess it depends on the kid. Most of them, though, have enjoyed their time in the classroom and were happy to interact with other prospective students. The writing/peer editing/rewriting process take up a good chunk of time, but since that's a rather lengthy process, that is probably to be expected. I think that in addition to academics, the school is looking at the social/emotional side of things, as well.
I do know that the school works very hard to get results back to families as soon as possible, which is no small feat. And I believe a detailed explanation of the assessment testing can be requested. The rest of the application process seems rather standard: test scores, recommendations, transcripts, writing samples, etc. If you have any questions at all about the application, definitely give them a call. When I was in your shoes I bombarded them with questions and it was never a problem.
I'm not sure if the way they did things last year is the way they'll do things in future (the school really is a work in progress), and, again, I have to stress that my family hasn't been through the current application process ourselves. Still, I hope this gives you at least some small insight into the assessment part of the application.
Good luck!
