using System;
class StudentGrade
{public static void Main()
 {Console.Write("输入学生百分制的成绩:");
    int Grade=(int)Console.Read();
    switch (Grade/10)
    {case 9:
     case 10:Console.WriteLine("你的成绩为:A");
            break;
     case 8:Console.WriteLine("你的成绩为:B");
            break;
     case 7:Console.WriteLine("你的成绩为:C");
            break;
     case 6:Console.WriteLine("你的成绩为:D");
            break;
     default:Console.WriteLine("你的成绩为:E");
            break;
    }
  }
 }

加入你输入了75……
那么75/10是多少啊……
你的case里面没有符合的那就只有default……

Console.WriteLine ("输入学生百分制的成绩:");
            int Grade = Convert.ToInt32(Console.ReadLine()) / 10;
            switch (Grade)
            {
                case 9:
                case 10: Console.WriteLine("你的成绩为:A");
                    break;
                case 8: Console.WriteLine("你的成绩为:B");
                    break;
                case 7: Console.WriteLine("你的成绩为:C");
                    break;
                case 6: Console.WriteLine("你的成绩为:D");
                    break;
                default: Console.WriteLine("你的成绩为:E");
                    break;
            }

            Console.WriteLine("Press Enter key to exit...");
            Console.ReadLine();