school.rb

class School
  attr_accessor :roster

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

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

  def grade(grade_input)
    @roster[grade_input] if @roster.keys.include?(grade_input)
  end

  def sort
    @sorted={}
    @roster.keys.sort.each { |grade_sorted| @sorted[grade_sorted]=@roster[grade_sorted].sort }
    @sorted
  end
end