"Baccano!"
It's...complicated. It's not set in a high school, it's set mostly in depression era america, with a focus on the incredibly violent passage of one train.
However, it's a non linear narrative that jumps between places, characters, and times, spanning decades at points. The main plot concerns rival crime families, a couple of petty thieves with a whacky MO, an insane hitman, a monster, and a bunch of immortals, to name only a few.
You'd be right in thinking it's hard to follow, and you'd be right. During the earlier episodes, at least. At this point, you have only a few pieces of the puzzle, but they are well chosen to be the most interesting ones, setting up the questions you would most wish to be answered, in a way a linear narrative never could, gripping you from the start. It even drops subtle clues in the intro and credits to things you won't even know about yet, and seeing everything come together later on is great, especially when it turns out to be the very opposite from what you expected.
From an entertainment view, it's enthralling, from an artistic and storytelling point of view, it's essential.