If they truly will be all the same programming, it could be as simple as a single computer running
powerpoint.
If you wanted it to be a complex conglomerated video
screen,
Screenly OSE is a free digital
screen system that would have 1 output and be split to feed all the TVs the same content, just like the
powerpoint solution.
Install instructions for a raspberry pi.
If there was the desire to make each
screen independent, Screenly has a paid version and there's many other harware based solution like Yodeck. You buy their device, put it on the internet, and configure from a website. All of these are subscription based.
Assuming the school will use these for entire school announcements, I'd recomend the single source solution, then in the future if they wanted to show live video or anything like that, there's no complicated middle software/hardware barrier, just direct wiring from a single source. I'd also recommend you install a sound bar at each TV. I assume they won't have audio from day 1, but one day there's going to be a video they want to
play with sound and TV speakers are trash at home, much less in a noisy high school.
For distribution, NDI makes the most sense. It's encoded video over
ethernet. If they have a robust existing gigabit
network, I'd use that. If it isn't robust, run your own
network cable. The nice part about NDI is its just packets on a standard
network. No
point to
point issues and uses regular
network equipment. For this distribution you'd get one NDI
Encoder box to put with the computer that's displaying the
powerpoint, and one NDI Decoder behind each TV.