grade.rb

module Packs
  module Admin
    class Grade < Grade
      # Stub class extending from the real definition
    end
  end # Admin
end # Packs

index.html.haml

%table
  %tr
  - for grade in @grades
    %tr
      %td
        = grade.name
      %td
        = link_to 'Show', resource(grade.school, grade) # This fails because grade.class.to_s is Packs::Admin::Grade rather than just Grade as the router is expecting
      %td
        = link_to 'Edit', resource(grade.school, grade, :edit)
      %td
        = delete_button(resource(grade.school, grade), 'Delete')

router.rb

 match('/admin', :sub => 'packs').to(:namespace => 'packs/admin') do |admin|
    admin.resources :schools, { :name_prefix => :admin } do |schools|
      schools.resources :grades do |grades|
        grades.resources :packs
      end
    end
    admin.resources :pack_products
  end