Hi,
It might be useful for others. I changed the design. Added a Manager domain object which extends AbstractParty. School and Region model does not extends AbstractParty anymore. And the policy entity definition looks like:
Hwwcn_Sponsor_Regions_Own: 'this.managers contains current.securityContext.party'
Hwwcn_Sponsor_Regions_Own: 'this.managers contains current.securityContext.party'
Hwwcn_Sponsor_Regions_Other: 'this.name != NULL &&! this.managers contains current.securityContext.party'
FYI: To make this work you will need to make the edit mentioned in How to add patch for review