The article states: "However, we found that fish in the centre of the school tend to react sooner than those at the periphery, in contrast with most previous work which found that fish at the edge of the school have a higher risk of predation " . This is an interesting outcome that does, as mentioned, suggest there might be other relevant factors in this.
Perhaps, a fish a the periphery is taking a greater risk by moving sooner, as it might then move out into open water. A fish in the middle of a small group can move while still remaining nominally part of the group, and if others move with it then it retains its central and safer position.
It would seem that in order to have a school of fish (as opposed to a melee for the safest position ) the group must display both poitional strategies; edge postion for successful feeding and mid position for quick evasion. What might happen to school organisation then if fish with a positional preference were selectively bred and kept together?