Ruby: School spec passing

school.rb

class School
  attr_accessor :roster

  def initialize(name)
    @roster = {}
  end

  def add_student(name, grade)
    @roster[grade] ||= []
    @roster[grade] << name
  end

  def grade(grade)
    @roster[grade]
  end

  def sort
    # Great explanation of how this works at
    # http://blog.jayfields.com/2008/03/ruby-inject.html
    # Thanks @ Chris Lee for posting this to Piazza
    @roster.sort.inject({}) do |hash, value_pair|
      hash[value_pair[0]] = value_pair[1].sort
      hash
    end
  end
end