I've taken a fair amount of lessons at a number of resorts know for challenge and steeps. Just to give you my bias and background, I'm about mid to upper level 8 skier and when I take lessons I prefer all day group lessons whenever possible. When I take a group lesson I'm often the only one at my level so I essentially get a private lesson for the price of a group lesson. However, in this situation some resorts will shorten the lesson, especially with half day lessons. I like all day lessons b/c it really gives you a good amount of time to learn something and you get a much more complete tour of the mountain. Realize that the quality of any lesson at any resort will be very dependent on your instructor and there's no guarantee that you'll get a good one. I always ask for a PSIA Level 3 certified instructor or higher and let the instructor assigning the lessons know what I'd like to learn, what I’d like to ski and what I'm working on. So here are some of my observations.
Alta: haven't taken a lesson there for a number of years (was probably a low level 7 skier at the time), had about 5 or 6 in my group that day, excellent instructors, only half day lessons starting in the afternoon, great way to find some of the famed Alta stashes, would like to go back and do their Diamond Challenge and really get to some of the steeps.
Snowbird: did the all day group Mountain Experience on a powder day (about a foot of fresh overnight), was more of a mountain tour than a lesson, about 4-5 in our group, did not like our instructor/guide as he was not too friendly or willing to teach, did a lot of traversing to get to the untracked, didn't really get to ski the classic Snowbird steeps, probably wouldn't do it again unless I knew I would have a better instructor/guide.
Jackson Hole: took a Mountain Masters all-day group lesson a few years ago on a powder day (8" of fresh overnight), had 3 in our group (1 was a buddy of mine), great instructor, great tour of the mountain, skied lots of the great JH runs except Corbet's, would do it again, possibly one of my all-time favorite days spent in a lesson.
Squaw Valley: took a 2 hour group lesson, was the only one in the lesson, decent instructor, skied mostly off Granite Chief, would've like to have skied off KT and Headwall, overall I'd say it would've been better if it would've been longer in order to get to other areas of the resort.
Crested Butte: took 2 half day group lessons during a week of skiing a couple of years ago, was the only one in both lessons, very good instructors, got to ski stuff that I probably wouldn't have been able to find on my own, skied all over the Extreme Limits terrain, having 2 half day lessons was probably better than having one all day lesson due to the differences in the instructors I had (both were very good, but had different styles of teaching), would do it again.
Big Sky: took 3 half day group lessons during a week of skiing a few years ago(was probably more of a mid to upper level 7 skier at the time), was the only one in all lessons, unfortunately only skied a couple of runs off the tram, mostly skied off Challenger and Thunder Wolf, decent instructors, would’ve like to ski more off the tram, in the future would probably take only 1 or 2 lesson in a week's time and ask to ski mostly off the tram.
Vail/BC: took 2 all day group lessons over the past couple of years, was the only one in each lesson, very good instructors, very good tour of the mountain, great to be able to cut lift lines especially at Vail, not too many true steeps but lots of excellent advanced terrain, would do it again.
Overall, I had my best lesson at Jackson Hole and am planning a trip back there this season to do the Steep and Deep Camp. I think my next best lesson experience has been at either Vail or Crested Butte. I’d love to get back to Alta for the Diamond Workshop. You probably can't go wrong with the EpicSki academy but I don't have first hand experience. If I was only going to take one lesson and then ski for a few days, I'd do it either at Jackson Hole or Vail. I hope this has been helpful.