public class FormDetailVM : INotifyPropertyChanged
    {
        public ObservableCollection<Student> Students { get; set; }
        public ObservableCollection<Student> Students2 { get; set; }
        public ObservableCollection<Student> Students3 { get; set; }

        private ClassForm classForm;
        public ClassForm ClassForm 
        { 
            get
            {
                return classForm;
            }
            set
            {
                classForm = value;
                
                if (classForm.FormData != null)
                {
                    FormData = JsonSerializer.Deserialize<ClassFormData>(classForm.FormData);
                }
                else
                {
                    FormData = new ClassFormData();
                }
            }
        }

        private SchoolYearClass schoolYearClass;
        public SchoolYearClass SchoolYearClass 
        {
            get
            {
                return schoolYearClass;
            }
            set
            {
                schoolYearClass = value;
                LoadStudents();
            }
        }

        public ClassFormData FormData { get; set; }

        public FormDetailVM()
        {
            Students = new ObservableCollection<Student>();
            Students2 = new ObservableCollection<Student>();
            Students3 = new ObservableCollection<Student>();
            FormData = new ClassFormData();
        }

        public void saveFormData()
        {
            using (var dbContext = new ApplicationDbContext())
            {
                dbContext.ClassForms.Attach(ClassForm);
                ClassForm.FormData = JsonSerializer.Serialize(FormData);
                dbContext.SaveChanges();
            }
        }

        private void LoadStudents()
        {
            Students.Clear();
            Students2.Clear();
            Students3.Clear();

            using (var dbContext = new ApplicationDbContext())
            {
                dbContext.SchoolYearClasses.Attach(schoolYearClass);

                foreach (Student student in schoolYearClass.Students)
                {
                    if (classForm.Student.Id == student.Id)
                    {
                        continue;
                    }
                    Students.Add(student);
                    Students2.Add(student);
                    Students3.Add(student);
                }
            }
            
            OnPropertyChanged("Students");
            OnPropertyChanged("Students2");
            OnPropertyChanged("Students3");
        }

        public event PropertyChangedEventHandler? PropertyChanged;

        private void OnPropertyChanged(string propertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

<ComboBox Grid.Row="1" Grid.Column="1" Height="30" Width="150" HorizontalAlignment="Left" VerticalAlignment="Top" 
   ItemsSource="{Binding Students}" DisplayMemberPath="FullName" 
   SelectedValue="{Binding FormData.Q1S1, Mode=TwoWay}" SelectedValuePath="Id" />