If you also consider the 'goals scored against opponents avg', this equates to possession and time not challenging the Keep. NW & Greenville's Keeps are rarely challenged in a game while the FM, SV & Wando Keeps are much more challenged.
School Rank - Goals Scored Against Opponents Avg
(1) NW: 6.8
(3) FM: 3.9
(4) SV: 3.0
(7) Wando: 3.7
(1) Greenville: 5.6
Stats alone, I'd give best to FM, SV or Wando!
