+ 1 for adding some danger to the tank, they would school more offten if they had a nice sized predator in the tank with them... For example, silver dollars will stay on the opposite side of the tank then a reddevil!!!!
think like a fish! they school out of fear and because they rely on each other to find food. I say increase the current in the tank and feed less. That should produce some results.
rummynose or lemon tetras school very nicely. I have 25 lemons that are always schooled. of course it helps to have 8 adult wild discus in the tank but the discus never show any aggression towards the lemons.
I might try some Cardinals. I like Lemons but I don't think they would look that nice in my tank with has brown substrate and lots of green plants. Any other Tetras?