I don't know about an absolute must for a planted aquarium, but there are plenty of schooling species I am particularly fond of:
Cardinal Tetras (P. axelrodi): the classic and most used schooling fish for planted aquaria. Not exactly tight schoolers, but they are very social fish that group together very tightly when they perceive a threat. They look spectacular in schools of twelve or more. Relatively hardy. They live about 2-3 years on average when well kept.
Rummynose Tetras (H. rhodostomus): probably the second most used schooling fish for planted aquaria (the fish I like most other people seem to like, too...). Excellent, tight schoolers with more subdued colors than the cardinal tetras. The brightness of their red noses is indicative of the water quality of the aquarium. There is also another species (P. georgiae) which grows slightly larger, but the nose is never as bright. A sensitive species --acclimate to your aquarium slowly.
Green Neon (P. simulans): lively, tiny little fish with a cool, green-blue neon stripe and less intense red on the belly than the cardinal tetra. Does not grow nearly as large as the cardinal (about half the size). Very social. Great for small planted tanks.
Red Phantom Tetra (M. sweglesi): not really a schooling fish, but still a very social tetra which tends to form loose groups. Their more ovate body shape and deep red color contrasts excellently with the more slender shape of the cardinal tetra. Not nearly as nippy as the serpae tetra.
Tube-mouthed Pencilfish (N. eques): another fish that forms loose groups. Very dainty, small species of delicate appearance. Not exactly very colorful but the unusual way in which they swim (at a 45 degree angle with the head pointing up) is very cool.
Silver Hatchetfish, Marbled Hatchetfish (G. sternicla and C. strigata): I have always loved hatchetfish! Unfortunately, since both of my current aquariums have openings in the back, I have been unable to keep any for a very long time. They will jump out of the tank given the slightest chance. The silver hatchetfish schools particularly well, in my experience.
Those are my favorites. I love most of the fish I've kept, really. Each seems to have its proper place.
Carlos