во

students/MainWindow.xaml

@@ -23,6 +23,6 @@
                 <DataGridTextColumn Header="Средний балл" Binding="{Binding average_score}" Width="*"/>
             </DataGrid.Columns>
         </DataGrid>
-        <Label Content="Средний балл группы: 4.34" Grid.Row="1" FontSize="20" HorizontalAlignment="Left" VerticalAlignment="Center"/>
+        <TextBox x:Name="average_score" Text="Средний балл группы: " Grid.Row="1" FontSize="20" HorizontalAlignment="Left" VerticalAlignment="Center"/>
     </Grid>
 </Window>

students/MainWindow.xaml.cs

@@ -31,6 +31,7 @@ namespace students
         private void Load()
         {
             studGrid.ItemsSource = _context.stud.ToList();
+            average_score.Text += _context.stud.Average(s => s.average_score);
         }
     }
 }