I am at Red Cliffs Lodge right now, chilling out before our See Ya Real Soon (final night) dinner. It has been an incredible week, and I plan to share more soon. We have had amazing weather all week. All the suggestions are great. I would just say to pack minimally and not worry about dressing up whatsoever. It is an extremely casual trip. Some will get a little dressier tonight. Last night was the Junior Adventure night and some parents cleaned up a bit for dinner then too but still quite casual for most. Most of the trip the most I have needed is a very light windbreaker during the day (I have a rainjacket that folds up small). I liked having my hoodie for the sunrise hike in Sedona but otherwise did not really need it. A sweater at night was nice at times. A fleece vest would have been a good option.
Otherwise remember to bring lotion and lip protection! The dry air and dust really dries you out!
I packed in packing cubes which I love but I wish I had packed more in individual outfits for each day in baggies so repacking would be easier. We did do one load of laundry here at Red Cliffs yesterday as we had reworn as much as we could.