That was quite an interesting story. Imagining such a smoke alarm in a dark room seems pretty uncanny to me as well.
Although i'm no fire alarm expert, I'm guessing maybe it was an older model Simplex smoke detector as you said everything else in the building were Simplex branded alarms.
What year was this school built?
I did a whole bunch of googling trying to find the answer here for you but nothing specific could be found. I do know however that the school district and local high school of the area has yearbooks spanning back to 1897, so chances are it's been around for quite some time!
I remember I started there in 2001 and entered just as the school had begun some modernization, a lot of it appeared to be very old pre 1980's stuff. The school bell was changed from mechanical to electronic and part of the intercom system the year I started, and the cool vintage old playground was demolished and reconstructed too. For the next four years, it underwent a visual transformation to look more streamlined and artistic, less like an old brick square I suppose.
Last I remember, there were Simplex 2901-9833+2903s, and Simplex 2098-9642s making up the bulk of what the system used if that helps!
When I worked as a summer camp counselor in another local elementary school in 2015 I did see that their old system had been revamped completely, so I imagine the other schools were still in the gradual process of modernizing their systems too.
As promised, here are the photos:
It's more yellowed than it looks on the pictures, the camera doesn't really render justice but yeah you can still see the chamber's openings. Also this detector is a Photoelectric/Thermal combo smoke.
On this photo, my hand serves as a scale to show how tiny it is compared to my hand, but it's actually much bigger than a modern-day smoke detector.
Holy cow that is one truly impressive monster of an alarm! It looks like it'd be really heavy.. the one I recall seeing did look very similar to this in regards to the color and style, I bet this is about the same time period of manufacturing. That's awesome to see. If you can imagine something like this alarm, but the circular part of it being about twice as large in circumference and extending downwards by a ridiculously long amount, almost like a cylindrical Smokegard 700 even, then that's pretty much it exactly.