It might help to distinguish  between 
 
a)  a   historiographic  approach  to  defining schools: the 
historian  of  economics  referring  to  a particular school 
would  have  to  use  -  implicitly or explicitly - a set of 
criteria  to  count  an individual to a particular school or 
not 
 
b)  a  sociological  approach:  observation  of  the  actual 
symbolic  practices of economists when they refer to schools 
(to the extent that they do); sb would belong to school X if 
they  were  regarded  by others (and/or themselves) to do so 
(allowing  for  the  case where there is controversy on this 
issue);  one  could  operationalise  this  by  investigating 
invocations of expressions referring to school X empirically 
 
a)  and  b)  are  not mutually exclusive, but arguably, most 
historians  of economics strive for a conceptual rather than 
sociological  definition.  One  should  note that putting b) 
into practice will rarely uncover a conceptually homogeneous 
body  of  writings. This reflects the seeming curiosity that 
whenever  a  particular  school  is  probed  for consistency 
across   individuals   one   tends  to  uncover  significant 
degrees of heterogeneity. 
 
Matthias Klaes