What is the ATC protocol for hijackings?
In the US, the ATC protocol for hijackings is classified for reasons of national security.


  10-2-6 Hijacked aircraft
  
  Hijack attempts or actual events are a matter of
  national security and require special handling. Policy
  and procedures for hijack situations are detailed in
  FAAO JO 7610.4, Special Operations.‡ FAAO
  JO 7610.4 describes reporting requirements, air crew
  procedures, air traffic procedures and escort or
  interceptor procedures for hijack situations


Air Traffic Organization Policy, FAA, 2015-12-10. 

‡This document's content can only be accessed from within the FAA network.