I think that 'artistic thought' about photography - even though it was respected at the time - was such that it would preclude there from being any 'school' of abstract expressionism in photography- except to consider those, in retrospect, like siskind. Siskind and a handful of others wanted to emulate it - but a photograph could never, and by any rigid definition, CAN never be 'abstract expressionist' insofar as it was much more about he isolating or capturing of GESTURE, by the canvas. Any photograph which can capture a given 'gesture' or movement requires it's author to remain perfectly still to record it (i.e. camera on a tripod, etc) - and any photographer attempting to record gesture via the imprinting of light on the negative - would, in most cases, only manage to capture a blurry morass on the negative. I'd assume it just wouldn't fly - to most contemporaries of the movement. Not without using paint or some other 'hard worked' medium. Just a thought.