This is a fairly common line of questioning I get from AHJs and even fire alarm technicians.
When an area of the building is renovated, or added on, the engineer would have needed to have that section meet the current requirements of the building code for the fire alarm system, but unless they or the AHJ determines some reason as to why the entire system needs to be brought to current code, only those sections of the building that fall under the building permit would be upgraded.
The main requirements regarding visual signals is found in the Ontario Building Code 184.108.40.206.(4) which states:
(4) Except as permitted by Sentence (6), visual signal devices shall be installed in addition to audible signal devices,
(a) in a building or portion of a building intended for use primarily by persons with hearing impairment,
(b) in a public corridor serving a Group A, B, C, D or E occupancy,
(c) in a corridor used by the public and in a floor area or part of a floor area where the public may congregate in a Group A occupancy,
(d) in not less than 10% of the suites of a hotel or motel,
(e) in a washroom for public use described in Sentence 220.127.116.11.(2), (3), (4) or (6), and
(f) in the living space in a suite of residential occupancy in a Group C major occupancy apartment building.
The code then goes on to discuss a few more requirements/exceptions in Sentences (5) and (6)
(5) Visual signal devices are permitted to be installed in lieu of audible signal devices in the compartments referred to in Article 18.104.22.168.
(6) Visual signal devices required by Clauses (4)(b) and (c) are not required in,
(a) a classroom, and
(b) a Group B, Division 3 occupancy that contains sleeping accommodation for not more than 10 persons and not more than six occupants require assistance in evacuation in case of an emergency.
A couple of other locations are found in: 22.214.171.124. Audibility of Alarm Systems
(7) Fire alarm audible signal devices shall be supplemented by visual signal devices in any floor area in which,
(a) the ambient noise level is more than 87 dBA, or
(b) the occupants of the floor area,
(i) use ear protective devices,
(ii) are located within an audiometric booth, or
(iii) are located within sound insulated enclosures.
(8) Sentence (7) shall also apply in an assembly occupancy in which music and other sounds associated with performances could exceed 100 dBA.
Essentially, 126.96.36.199.(4)(b) would have required visual signals in the corridors that were renovated, and (e) for universal washrooms, but the remainder of the building would have been permitted to remain as is unless the engineer designed the system to be entirely renovated and upgraded in the building.
Of course, (and as always) the AHJ gets the final determination of what is deemed acceptable or not, so the plans reviewer and the CBO for the municipality would have made the call on what, if anything they wanted to see upgraded in the school. There may have been a reason they didn’t make them install the visual signals in the renovated areas.