[Serializable]
public class Student
{
    public string Name { get; }
    public string LastName { get; }
    public int GroupNumber { get; }
    public List<int> Grades { get; }

    public Student(string name, string lastName, int groupNumber, List<int> grades)
    {
        Name = name;
        LastName = lastName;
        GroupNumber = groupNumber;
        Grades = grades;
    }

    public static Student Create(string studentInfo)
    {
        string[] data = studentInfo.Split(' ');
        string name = data[0];
        string lastName = data[1];
        int groupNumber = int.Parse(data[2]);
        List<int> grades = new List<int>();
        for (int i = 3; i < data.Length; i++)
        {
            grades.Add(int.Parse(data[i]));
        }
        return new Student(name, lastName, groupNumber, grades);
    }
}
public static class Deserializer
{
    public static List<Student> DeserializeStudents(string path)
    {
        // При десериализации файла необходимо прописать FileAccess.Read
        List<Student> students = new List<Student>();
        BinaryFormatter bf = new BinaryFormatter();
        using (FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read))
        {
            students = (List<Student>)bf.Deserialize(fs);
        }
        return students;

    }
}
public static class Analytics
{
    public static double FindGPA(List<Student> students)
    {
        double sumOfAllGPA = 0;
        for (int i = 0; i < students.Count; i++)
        {
            Student student = students[i];
            int sumOfGrades = 0;
            for (int j = 0; j < student.Grades.Count; j++)
            {
                sumOfGrades += student.Grades[j];
            }
            sumOfAllGPA += Math.Round(sumOfGrades * 1.0 / student.Grades.Count, 2);
        }
        return Math.Round(sumOfAllGPA / students.Count, 2);
    }

    public static void WriteStudentsWithGPANoLessThanAverage(IEnumerable<Student> students, string path, double GPA)
    {
        List<string> output = new List<string>();
        output.Add(GPA.ToString());
        foreach (var student in students)
        {
            int sumOfGrades = 0;
            int amountOfGrades = 0;
            for (int j = 0; j < student.Grades.Count; j++)
            {
                sumOfGrades += student.Grades[j];
                amountOfGrades++;
            }
            double localGPA = Math.Round(sumOfGrades * 1.0 / amountOfGrades, 2);
            if (localGPA >= GPA)
            {
                output.Add(string.Format("{0} {1} {2}", student.Name, student.LastName, student.GroupNumber));
            }
        }
        using (FileStream fs = new FileStream(path, FileMode.Create, FileAccess.Write))
        using (StreamWriter sw = new StreamWriter(fs))
        {
            foreach (var line in output)
                sw.WriteLine(line);
        }
    }
}