Chromis and Apogon leptacanthus are the only 2 readily available ones I can think of.
Schooling is tricky in a tank- if the fish don't feel any threat of predation eventually they will not bother with schooling. If you have a very large tank with some boisterous fish like tangs they might hang together.
I have 3 of the Apogon and they school somewhat in a 120. If it was a bigger tank with more of them it might be more impressive...
Just strolled into my lfs and there they were one day, getting pummelled by some big fish. One of them has a mouthful of eggs every couple of weeks. I have to say, in person their colors are subtle compared to most marine fish. The eye is beautiful tho, like a neon tetra.
Good timing for this thread. I just saw 3 green chromis at the LFS. They're beautiful but HUGE! (they're usually 10 bucks canadian)These were 35$ each. I too was afraid that their immense size meant that they could not be expected to live a long time. Does that make sense? They really were stunning, though.
Some people have had success keeping groups of banggai cardinals although I'm not sure that they would classify as a schooling fish.
OP - LiveAquaria.com is the only retailer that seems to advertise them regularly. I was thinking about getting 5 or so as a dither fish for my 215 gallon - 18.99 a piece they are not cheap!
There are also some orange striped cardinals that are really neat looking - another Apogon species.
If your tank is large enough a school of Bannerfish / heniochus is a stunning sight. Also I really like the coral catfish / Plotosus lineatus as a juvenile schooling fish but they get large, ugly and destructive as they grow older.
Perhaps a small harem of flasher or fairy wrasses might work too.