int grade=0, tests=0, student = 1;
double sum=0;
bool end = false;

Console.Write("Student {0}, Test {1} result: ", student, tests+1);
grade = int.Parse(Console.ReadLine());

while (!end)
{
    if (grade > 0) { sum += grade; tests++; } // grade ok, continue
    else if (grade == 0 && sum == 0d) { end = true; } // grade = 0, and no sum, end
    else // grade 0 start new student
    {
        Console.WriteLine("Student {0}, Avarage is: {1:F2}", student, ((sum > 0) ? sum / tests : 0));
        student++;
        tests = 0;
        sum = 0d;
    }

    if (!end)
    {
        Console.Write("Student {0}, Test {1} result: {2}", student, tests + 1, (tests == 0) ? "(or Type 0 to Exit)" : "");
        grade = int.Parse(Console.ReadLine()); // get next test result
    }
}

Console.ReadLine();