I could not remember the exact number you had, but I suspect it is a maturation thing and lack of personal space within the school. Someone of the web used the comparison to a cluster of bubbles, each fish spacing itself with its bubble in contact with its schoolmates bubbles. We can extend that idea with the fact that when you compress bubbles, the internal pressure goes up - in this case that is released by nipping. I think your problems will ease markedly when they move.
In the meantime, can you reduce the light, possibly also the temp a couple of degrees? Anything along that line might give you a bit more margin.
The minimum number of fish per captive school seems to be highly specific. I do say 8 for Tiger Barbs, but I don't know enough to say for the Sebaes.
HTH