Most passenger cars have a roof load of 50kg. The roof load for the respective vehicle is stated in the vehicle registration document. For your driving school vehicle it is 75kg. Please note that you need an appropriate roof rack, as the panoramic roof (glass roof) cannot be loaded directly.
You must secure the luggage or items so that they cannot slip or fall while driving. For this reason, the use of roof boxes is recommended. There are now appropriate roof racks for many items (skis, snowboards, bicycles, etc.).