In lakes, once the water hits temp low to mid 70's muskies will move to deeper water in search of cooler conditions. Musky also require alot of food, so fish will congragate into an area with abundant baitfish and likeable temps. In river systems it is rare to see more than 2 fish together although I have hooked up with 5 in a day out of a small area before which was probably for the same reason the fish group described above. In lakes of high population however, it is not uncommon to mark groups of 10 or more musky in deep water near pods of bait during mid and late summer. So I don't think that it is schooling so much as it is numbers of fish inhabiting the same area because of favorable conditions.