Domain Model for a School day - 12 homework

school.rb

class School
  attr_accessor :roster
  def initialize(school_name)
    @roster = {}
  end

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

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

  def sort
    self.roster.keys.each do |grade|
      self.roster[grade].sort!
    end
    self.roster
  end
end