I'm also curious if this is more a pond thing than lake.
Once the water warmed in my pond, about 20 or more sunnies were cruising just below the surface in a large school swimming in the same direction parallel to shore. A bass was swimming beneath them. It was a bright sunnie day with wind around 5-10 mph, water temperature I'm sure warmer near the surface at about 50.
Have no clue the biology of this almost daily visual.
I see this fairly often early in the year as water starts to warm in small lakes and ponds. Surface temps are a little warmer then. Those cruisers rarely eat my bait but if one does get hooked the bass often will attack the hooked fish.
This is my favorite time of year to fly fish for bluegill in ponds.
:popcorn
I have seen a school of what turned out to be white bass doing this in a creek arm of a large lake. Huge school. Took about 5 minutes to completely swim by the spot I was fishing. was doing laps in the area it seamed. they would swim by about every 30-45 minutes. Didn't want none of my offerings though. Was cool to see though...
seen young crappie do it small waters before , but not sunfish , I imagine they are bring "herded" by the black bass to concentrate his targets , not totally sure though :dono
I've seen it often but usually in early to mid summer, not early season. I've tossed a lot of baits at them with nary a hit or hookup. I just know they're laughing at me for trying when I see them schooled up like that. I know they're not gonna bite but I gotta try anyway.
I'd call my fishing holes ponds, not lakes for size reference.