This was added in IBC 2012:
[F] 907.2.3 Group E. A manual fire alarm system that initiates the occupant notification signal utilizing an emergency voice/alarm communication system meeting the requirements of Section 907.5.2.2 and installed in accordance with Section 907.6 shall be installed in Group E occupancies.
Group E = educational occupancy, or schools. This section of the code mandates any schools built under IBC 2012 or newer have voice systems.
Now the major city I currently work in has only adopted IBC 2009, so depending on where you are it may or may not be a requirement yet. This also only applies to new construction (or massive renovations), and even if the building is built in 2017 in a city with 2012 codes adopted, if the plans were approved in 2010 under 2009 codes it’s still valid to not have a voice system. And that, is the fun with building codes…