Well Mike, you know what they say, if you can't define it you don't understand it.
I think this is an excellent question. What exactly was the Philadelphia School, who was included in this group and why were they included? There is no point in having the notion of a Philadelphia School floating about if nobody knows exactly what it was.
Well Sean, I'm simply desperately trying to avoid getting sucked into another GCA black hole.
To me the "Philadelphia School" is simply a name that someone coined that tried to put some definition and categorizaton around the fact and phenomenon that a loose-knit bunch of avid, amateur, competitive golfers from the Philadelphia region shared some degree of mutual common interest (in promoting the development of the Philadelphia competitive golfer) and friendship that led them to actual collaborative design efforts which resulted in the creation of some of the best golf courses in the region and the world.
If there was a champion, it was likely Tillinghast, who used his power of the printed word to rail against the poor designs, kept everyone abreat on new developments, and then later participated in his own design efforts as well as probably working with everyone from George Crump to George Klauder.
This model probably started with the "Committee" design of Merion, which although led by Hugh Wilson involved outside advice from Macdonald/Whigham and a 5-man panel of amateur golfers who brought various skills to the job. It continued at Pine Valley, as George Crump sought input from all of the top Philadelphia golfers and designers, but also from guys like Travis, Benny Sayers, George Thomas, etc.
It was observed in other course settings, as well, where Hugh Wilson, Ab Smith, and William Flynn helped J. Franklin Meehan redesign North Hills, or where Hugh Wilson worked with Herman Strouse of Philmont to add new holes and wholly revise others, or Tillinghast working with George Klauder and Cecil Calvert on the old Aronimink, although the former was likely more designer while the latter two were likely more constructors. In that spirit, they also had some advice from Vardon & Ray that contributed to a hole or two of that course.
It may have achieved it's zenith with the creation of Cobb's Creek, somewhat ironically.
At the time it was created, Robert Lesley appointed not a professional golf architect to the job, but instead a committee of the best amateur sportmen in the area, all of whom had design and/or construction experience at their own clubs, and included Hugh Wilson, George Crump, Ab Smith, George Klauder, & J. Franklin Meehan. Dr. Father Carr, William Sargent, and George Thomas had involvement, as well.
It was a model that obviously worked well, but was soon supplanted by the advent of the professional architect.
Direct spawns of these early efforts included George Thomas and William Flynn, and later Tillinghast's more dedicated, focused design career.