Our kids can branch out and go to whichever parks they want. They do not travel en masse.Considering that the group tickets for my group are $331 per person for 4 day, no Park Hopper as opposed to $556 regular price for the same dates, I'm okay with it.
I don't think the reasoning is based on any single group that is there. But...considering that at the time that my group is there, I know of 4 other school music groups going just from Maryland. Between us, it's about 700 people. Looking at the performance reservation calendar, I'd say they have roughly 30 high school music groups there over MLK weekend. The potential swing of 5000 or more people from those groups could have a significant impact. Not just on park attendance an attractions, but on bus parking, food service, etc.
I have also been led to believe that when multiple large school groups are in a park, they adjust the number of security in that park. Not sure if that is accurate, but the information comes from a tour company that specializes in HS group trips to WDW, so I'm inclined to believe it.