IMHO, this is a fault of the student more so than having a group.
I would listen to the instructor tell someone to shorten their reins, or to lift their toes and settle back - and then I would watch to see if there was any reasonable attempt at complying.
And in many cases, the answer was no - which is why some of the riders seem to be riding at the same level they were when I took my first lesson there.
There are things that can be taught in a group that cannot be taught individually, and vice-versa. But no one can be taught if they won't listen.
I am going to disagree. I think this falls on the Coach. Many Coaches are not capeable enough of teaching 5+ students at a time, and in such "large" groups, each student does not get the needed attention to help them obtain what it is being given to them, and to apply it. The coach has to spread themselves over 5+ riders in the hour of the lesson - that's less than 15 minutes per indavidual that the rider/student gets from the coaches attention.
In a Private situation, the Coach can focus entirely on that student, and to help them obtain what it is that they are trying to teach them. If the Coach doesn't see them allowing their heels absorb their bodies weight, then they can focus on that issue until the student's light bulb goes off. Instead, in a group lesson, that Coach doesn't have the needed time to find that switch - because they have to focus on 4+ other people who are riding with him/her at the same time.
It's one thing to be told to get your toes at a 45 degree angle, but it is another to understand as to why, how and what it will do for your lower leg if you do apply it.
In the group lessons I would observe taking place, I found the Coaches would watch one rider do their course, throw out a few "pointers" and then say "Good job" and then the next would go, while that student would sit there for 10 minutes until it was their turn again. I would see these students not improving in the least and if they did, it wasn't very much. Reason being, is because the Coach cannot possibly give each indavidual the needed attention time.
There is no way, that a Coach can give each indavidual rider what they need/require in a group lesson - while in a Private, that indavidual rider gets the full attention of that Coach for the whole lesson.
In groups, I do not obtain anything when the Coach shouts out at me to fix something, and then their attention is moved onto someone else, leaving me to my own devices until she/he chooses to direct their attention back at me. All the while I am trying to figure out if I am doing what it is, correctly or not.
If groups work for you, fabulous. That's great. Stick to them if that's what works for you.