Fish typically don't school unless they're stressed or scared. Unless you want them to be stressed by putting a predator in there, it's best to appreciate that they're content and not threatened.
That said, I don't know what your numbers are or if you have room, but if you're looking for more activity you could try adding a small school of barbs. Rosey, Odessa, etc. NOT tiger or tinfoil. But IME most barbs are more boisterous and will instigate a bit more shoaling activity from the other fish.