C# Blocking specific types of answers using a sentinel value

I am writing a code to get the highest mark and lowest mark. It also collects the average. It uses 999 to exit out of the do loop. I need to get it to stop adding the invalid answers as well as the 999, but I can't get it to work. (mark != 999) stops 999, the exit command, from being added to the grades we're trying to track, but I need to make it so it blocks marks above 100 or below 0 and I can't seem to get it to work.

The block of code I am working with is this:

    if (mark != 999 && (mark < 0 || mark > 100))
                {
                    sum += mark;
                    count++;
                }

This is the main program:

    int mark,
            sum = 0,
            lowMark = 100,
            highMark = 0,
            average,
            count = 0;
        char playagain = 'N';
        // In a do loop, ask the user to enter a grade for a student or 999 to quit
        do
        {
            do
            {
                Console.Write("Please enter a mark for the student or enter 999 to quit: ");
                mark = int.Parse(Console.ReadLine());
                while (mark != 999)
                {
                    if (mark != 999 && (mark < 0 || mark > 100))
                    {
                        sum += mark;
                        count++;
                    }
                    if (mark < lowMark)
                    {
                        lowMark = mark;
                    }
                    if (mark > highMark)
                    {
                        highMark = mark;
                    }
                    if (mark < 0 || mark > 100)
                    {
                        Console.WriteLine("Invalid input value.");
                        Console.Write("Please enter a mark for the student or enter 999 to quit: ");
                        mark = int.Parse(Console.ReadLine());
                    }
                    else break;
                }
            } while (mark != 999);

            average = sum / count;

            Console.WriteLine($"\nThe class average was {average}%");
            Console.WriteLine($"The highest mark was {highMark}% and the lowest mark was {lowMark}%");

            Console.WriteLine("\nWould you like to start again?: Y/N");
            playagain = char.Parse(Console.ReadLine());
            playagain = char.ToUpper(playagain);
        } while (playagain == 'Y');
        // In a while loop, the program will:
        // - Determine if the entered mark is the highest or lowest grade
        // - Validate the entered grade before entering another grade; display some error
        //   message if the grade is invalid (i.e. < 0 or > 100)
        // - Prompt the user to enter another grade or 999 to quit

        // If there are valid marks:
        // - Calculate the average grade for the class
        // - Display the average, highest, and lowest grades
        // Else, display some error message
        // - Prompt the user to redo the steps above or quit.