IME, the very best schoolers are Debaui catfish, but they are only diurnal in less-than-bright tanks unless there is a good cover of floating plants - without that they go nocturnal.
Next best are slender Rasboras, the Green-eyed Rasboras are excellent small fish, the Scissors-tailed Rasboras for medium, multiple larger Rasboras are excellent schoolers also. The Rummy-nose Tetras are the best of the Tetras, but Bleeding Hearts will come close in larger tanks with good open swimming space across the front of tanks >4' in width. Ditto the Congos for larger tanks (but they may well nip tender new growth on stem plants).