It is possible to address individual rooms quite simply on 70V line.
For multi-channel, each speaker has a rotary switch connecting it (and
its transformer) to different 70V lines. This used to be used for
multichannel radio to hospital and hotel bedheads.

Simpler is to have a single channel setup, with switch / volume control
on the speakers to turn them off when the programme is not wanted in
the classroom. If an additional pair of wires is run, and a relay is
used at each speaker, all speakers can be turned to maximum for
all-call and emergency announcements, class change pips, etc. Google
for 'volume restoration relay'.

Owain