sample.rb

----------------------------------------------------------------------------------------------追加箇所↓
def int_valid?(string)
  Integer(string)
  return true
rescue ArgumentError
  return false
end
----------------------------------------------------------------------------------------------

class Score
  # 生徒のスコアを保存しておく配列
  @@lists = []

  def self.input
    puts "生徒の名前を入力してください"
    name = gets.chomp
    puts "得点を入力してください"
----------------------------------------------------------------------------------------------追加箇所↓
    input = gets
    while int_valid?(input) == false
      puts "入力された値は無効です"
      puts "得点を入力してください"
      input = gets
    end
    score = input.to_i
----------------------------------------------------------------------------------------------
# aという変数に判定の返り値を代入
#     a = score.match(/\d/)
#     while a == nil
#       puts "入力された値は無効です"
#       puts "得点を入力してください"
#       score = gets.chomp
#       a = score.match(/\d/)
#     end
#     score = score.to_i


    data = Score.new(name, score)
    @@lists << data
  end

  def initialize(name, score)
    @name = name
    @score = score
  end
  attr_accessor :name, :score

  def self.show
    # 結果表示
    line = "------------------------"
    puts line
    total = 0
    count = 0

    @@lists = @@lists.sort{|a, b| b.score <=> a.score }

    @@lists.each do |list|
      puts "#{list.name} : #{list.score}"
      total += list.score
      count += 1
    end
    average = total/count
    puts line
    puts "平均得点 : #{average}"
  end
end

while true do
  puts "得点を入力しますか？ : [0]Yes [1]No [2]Quit"
  input = gets.to_i
  if input == 0 then
    Score.input
  elsif input == 1 then
    Score.show
  elsif input == 2 then
    exit
  else
    puts "入力された値は無効です"
  end
end