public async Task AddStudentsToClassAsync(Class class1, string[] studentsid)
        {
            if (class1 == null)
            {
                throw new ArgumentNullException("class1");
            }
            if (studentsid == null)
            {
                throw new ArgumentNullException("studentsid");
            }
            var list = new List <User>();

            foreach (var student in studentsid)
            {
                var user = await UserStore.FindByIdAsync(student);

                if (user != null)
                {
                    list.Add(user);
                }
            }
            await UserStore.AddStudentsToClassAsync(class1, list.ToArray());
        }