Can hours at an FAA flightschool be credited to an EASA PPL?
Assuming that "flight school hours" means flight time then CAP 804 suggests yes:


  3.6 Logging of Flight Hours gained in the USA   Some flight hour recording practices allowable in the USA do not comply with European
  and UK requirements. In particular:
  
  
  2 pilots flying together in a single pilot aircraft both claiming P1    hours; 
  One pilot accompanying another on newsgathering or traffic    control flights and claiming P1/PIC when they have not acted as Captain or signed for the aircraft.
  
  
  Flying hours of this nature cannot be credited towards the
  requirements for Part-FCL or UK Licences.


The last sentence implies that if hours logged in the US do comply with "European and UK requirements" then they can be credited towards licenses. If you need something more detailed and explicit then it may be easiest to ask the CAA directly.