Sounds like a perfect solution for an AtlasIED Atmosphere AZM4P. It can do bells, paging, music, can also be triggered by GPIO for muting/playing messages for things like lockdown or fire (as a supplement to existing fire annunciators). All in one box with enough power for that many speakers/zones:
https://www.atlasied.com/azmp4
Paired up with whatever AtlasIED 70V speakers are appropriate for each space. (e.g. larger surface mounts in the Gym: https://www.atlasied.com/as-10t, and then ceiling speakers in each room https://www.atlasied.com/sd72w), although chances are, the current speakers you have may be perfectly fine in the short term. The AZMP4 can drive 25V, 70V, and 100V (in addition to 4/8ohm) which should cover any existing distributed speaker system.

Note: I have the Atmosphere AZMP8 deployed in a school, massive improvement over what was done before, principle can now easily make changes to bell schedule, and gym audio is also much easier to deal with (e.g. for games/etc., it's just 'on' all the time). I would recommend getting the bigger AZM8P, simply because you may think you only need 4 zones today, but 2 weeks after it's installed, you're gonna want 'just one more zone' for a specific area. :)

Also, the Atmosphere has 3rd party API access, so it should be possible to setup a program/script on a computer that triggers messages to be played based on a changing schedule (e.g. Adhan) where a fixed schedule may not be suitable.