they are determined by whichever group--the socs or the greasers-- run away, in which this case, the greasers win.
This winners of the rumble were determined by who ran first. It is true that a member of Tim Shepard's gang did break the rules but that was not a determining factor. The point of a rumble was to show who was toughest, who could take more. By running, the opposing gang was admitting defeat.
The winners were determined when one group ran away.
The greasers ended up winning when a member of Tim Shepard's gang uses a metal pipe, which was against the rules.