Loader::model('page_list');
$pl = new PageList();
$pl->ignorePermissions();
$pl->includeSystemPages();
$pages = $pl->get($itemsToGet = 100, $offset = 0);
$allPagePermissionKeys = PermissionKey::getList('page');
$allPagePermissionKeyHandles = array_map(function($permissionKey) {
 return $permissionKey->getPermissionKeyHandle();
}, $allPagePermissionKeys);
foreach($pages as $page){
  $page->clearPagePermissions(); // reset to nothing
  $page->setPermissionsToManualOverride();
  // Add permission for the Administrators group to do EVERYTHING
  $page->assignPermissions(Group::getByName('Administrators'), $allPagePermissionKeyHandles);
}