gistfile1.rb

class GradeLevel
  attr_accessor :number
  def initialize(num)
    @number = num
  end
  def order_index
    number =~ /K(\d)/ ? $1.to_i : (number.to_i + 2)
  end
end

def short_string(collection)
    # as_i = collection.all.map{|x| x.order_index}.sort
    as_i = collection.map(&:order_index).sort
    
    collection.sort_by(&:order_index).slice_before {|x| !as_i.include?(x.order_index-1)}.map {|arr| "#{arr.first.number}-#{arr.last.number}"}*", "
end

grades = ["K0", "K1", "K2",  "4", "5", "6", ].map {|num| GradeLevel.new(num)}
short_string(grades)

class GradeLevel
  attr_accessor :number
  def initialize(num)
    @number = num
  end
  def order_index
    number =~ /K(\d)/ ? $1.to_i : (number.to_i + 2)
  end
end

grades = ["K0", "K1", "K2", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12"].map {|num| GradeLevel.new(num)}
short_string(grades)

grades = ["K0", "K1", "K2",  "4", "5", "6", ].map {|num| GradeLevel.new(num)}
short_string(grades)

def order_index(number)
  number =~ /K(\d)/ ? $1.to_i : (number.to_i + 2)
end

def short_string(collection)
  #in this case, we're calling order_index with an argument, not as a method on an object
  as_i = collection.map {|num| order_index(num)}.sort
  collection.sort_by {|x| order_index(x)}.slice_before {|x| !as_i.include?(order_index(x)-1)}.map {|arr| "#{arr.first}-#{arr.last}"}*", "
end

will also work