school-domain.rb

class School
  attr_accessor :name, :roster

  def initialize(name)
    @roster = {}
    @name = name
  end

  def add_student(student_name, grade)
    self.roster[grade] ||= []   
    self.roster[grade] << student_name
  end

  def grade(grade)
    self.roster[grade]
  end

  def sort
    self.roster.each do |key,value|
      value.sort!
    end 
  end

end