I like schools of nanofish, even in larger tanks. My current favorite is
Microdevario kubotai, neon green rasbora. Hardy, easy, friendly, not shy, pretty, look gorgeous in a school and school nicely, esp in a planted tank. Not sure why they don't get more 'press'.
I hear great things about Glowlight danios (Danio choprae) as tightly schooling fish, but I've never tried them.
I also have a group of ~12 celestial pearl danios (formerly galaxy rasbora; Celestichthys margaritatus). They don't 'school' and are generally shy, but their behavior is fun to watch the way the males show off circling each-other. They are certainly not shy when I come up to the tank around feeding time.