Plainfield North isn't that far from Benet or Marmion (13, 16 mi respectively vs. 18 for Provy taking 30 then crossing over)? As for LWN kids, people dont cross the river to go JCA that often.
Also, a lot of Plainfield North kids are were among the first kids in their subdivision (I have a nephew who's a varsity DB for PN - a kid with state title pedigree from his mom's side uncles and a grandad who played for Bernie OBrian), so there's a sense to go where their parent's taxes paid for.