How many tetras do you need to school?

will tetra's school with differnt species of tetra or just with their own kind?

in my 50G i have 5 serpae, 2 flame tetras and 1 lone glassfish that all pretty much shoal around together like little "homeys". but not in a tight school. i have read (Aquarium Fish Magazine, March 2007 issue) that different tetra species will do that.
 
I have 5 scissortail rasboras and 2 lemon tetras that "hang out" together, but really don't zip back and forth in a school per se. When they are scared, they go every which way, not together... It's funny to watch all of them scatter, then come back together in a bunch.

I also have 5 neons that bunch together (however, there is one that usually doesn't--the outcast maybe). They don't hang with the bigger fish though, but when they are scared, they do all go in the same direction, then settle down again.

Like the others said..schooling is a defense mechanism.
 
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