Here are my theories:
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I’ve seen large grilles like the one in the first picture. They could have bells or horns, and in some cases, both. There are other components in your school (i.e., pull stations, detectors, panel, older/disconnected devices) that could give clues as to what the original system was. Even old clock systems are sometimes indicative of the brand of an old alarm system (i.e., Simplex, Edwards, Standard Electric). Do you know of any other schools in your area with similar setups like the ones at your school, or what brands they use or have used? Sometimes, old hardware could still be concealed behind those grilles, but you may not want to arouse suspicion in case of surveillance cameras or anybody nearby.
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I’m betting those fire alarm signs are insights into one or two things: Your school may have had a manual pull-rod system tied in to the electronic alarm system, or those signs may serve as indicators of a pull station underneath. It’s also possible there could be a wall scar or metal plate instead, indicating that there was an old pull station. If a wall scar is round, for example, there may have been a Standard Electric pull station there at one time.
Although they’re not always the best quality, old pictures from your school’s archived yearbooks are also great clues. You may be able to find them at your local library.