Sven,
As far as I can tell, the system was a one-way 100V system, using a matching transformer and volume control at each speaker.
The volume control wasn't a standard potentiometer, but it was a 6-position switch connected to various resistors.
Now that I come to think of it, in the position with the least resistance, you'd only need a speaker in every second room, because Man was it LOUD!!. [Linked Image]
These days if there was something that loud in a classroom, the kiddies would all have to wear OSH-approved ear protection, National Anthem or not!. [Linked Image]