On the last day of summer break, Celine Chien stood by a giant wall at Menlo-Atherton High. Paintbrush in hand, the rising junior faced a half-finished mural three times her height — one Celine had designed after taking an ethnic studies class in ninth grade. On one end, there was a group of Native Americans from the Ohlone tribe, the Bay Area’s original inhabitants. On the other, a group of high school students stood together, eyes toward the future. A series of characters filled the gaps in between, from immigrants sprinting toward the Gold Rush, to protesters brandishing Black Lives...