I work at Stowe as an instructor, and there are a few others on this forum who do as well. We have a large number of Level 3 and Level 2 instructors. Stowe prides itself on being the oldest ski school in the country, and we have the experience to show for it. Instructors with 20+ years at Stowe are very common.
As far as a full day group lesson, it depends entirely on the age of the people you're talking about. Our children's group programs are all full day programs, and those run ages 3-14. Group size depends on age. Adult groups are 2 hour lessons, and groups are allowed to be larger than kids group. However, it all depends on when you go. If it's a midweek in January or March, you're likely to get a group with only one or two other people. If you go MLK or President's weekend, you're more likely to have a large group.
You can sign up for a semi-private, which gives you much more control over who you get as an instructor and of course who is in your lesson.
I have worked at a few mountains in the northeast, and that is pretty typical for what is available around here. The simple reality is that the Northeast has far more people and far more skiers than out West, especially JH. More skiers means more people taking lessons, which translates to larger class sizes.