What you usually end up doing with your group of 10-20 skiers is group drills, which benefit each and all in varying degrees, while the feedback is more or less individual. The opposite is a specific individual feedback and individual drills.
The get better day is really something you can realistically do for one hour, but it can help each kid as much as an entire day of "group therapy".
The idea is fairly simple: you, the coach, sit in the middle of the slope and signal the kids to come down, with their best skiing (or maybe a drill) and they stop at you. You then give them individual feedback and then give them one of the drills for the second half of the run, then signal the next kid. They cycle on the lift, working on your last feedback, while you wait there.
Starting with the clear message that everybody gets better is very motivating. The kids will quickly pick up on the fact that they get very individualized feedback and drills and will get into it.
The one obvious issue is time management: you don't want a big pile of kids waiting at the top - having two coaches at the middle will help. If that's still not enough, you need to get creative and maybe send some on a directed run with a drill, before cycling back in line.
Taking notes helps keeping you consistent. It is a good idea to keep feedback and drills focused on one major item for each skier, the SMIM or Single Most Important Movement to Improve.
If you add warm-up and a few directed group runs, this will take perhaps two hours or half a day of a short program.