grade_school.rb

class School
  def initialize
    @school = {}
  end

  def add(name, grade)
      ( @school[grade] &. << name ) || @school[grade] = [name] # Yay 2.3.0!
  end

  def grade(grade)
    @school[grade] || []
  end

  def to_h
    sorted_names = @school.each_with_object( {} ) do |(grade, students), sorted|
      sorted[grade] = students.sort
    end

    Hash[sorted_names.sort]
  end
end