Well, where do you think the Germans put their U Boats after the war? They were still there in my time (only 25 years after the hostilities).
They were scuttled, and it was a thrice-yearly occurrence, it seemed, to see huge marine cranes in the bay dragging them up, presumably to be refurbished for some museum or old for scrap.
You can't see any in those photographs, but there were a number of huge two-storey bunkers with concrete walls about ten feet thick on the school property. They were cold, creepy places and had large enough interiors to be used as shooting ranges for the cadets.
They also had underground passageways that led to secret docking points for mini subs, something the Germans were developing before the end of the war.