Hi Angel

When we took Archie to training classes there was a GSD there who was very much like this.

He was a rescue and hadn't been properly socialised so for him the sight of another dog, let alone a room full of them, was about the most exciting thing he had ever seen! His behaviour was exactly as you described it.

What they did in the classes was to allow him in, on a lead, controlled by his owner. If he started barking his owner would take him outside, only bringing him back in when he was calm. The GSD learned pretty quickly that he had to stop barking and prancing around, and after a few weeks I could not believe it was the same dog, sat quietly in a corner with his owner.