Well, they’re not entirely yellow (the head is a yellow-gold, and the body a rosy pinkish color), they’re terrible for shipping and acclimating, and they’d likely have to be special ordered from a company that hand collects specimens, but my nomination is the golden sweeper (Parapriacanthus ransonneti). If you get them acclimated, they’re supposedly pretty hardy, they definitely school, and they really make a statement in an aquarium.
Alternatively, you could do the ring-tailed cardinalfish (Ostorhinchus aureus). They’re nocturnal and more of a coppery color than vibrant yellow, but they’re cool little fish, and I’ve heard that they should school/shoal pretty well (full disclosure: I have no idea how they’d do long term in a group - they might eventually kill each other like a lot of other cardinals do, or they might be fine).
Otherwise, the other suggestions given earlier of things like yellow tangs and foxface rabbitfish are probably your best bet if you’re set on the schooling and not just movement in various areas of the tank. There’s my two cents for you.
