public async Task SaveChangesAsync()
        {
            var tasks = new List <Task>()
            {
                Task.Run(() => Lessons.Save()),
                Task.Run(() => Students.Save()),
                Task.Run(() => Subjects.Save()),
            };

            await Task.WhenAll(tasks);
        }
 public void SaveChanges()
 {
     Lessons.Save();
     Students.Save();
     Subjects.Save();
 }