The main reason that organizations choose commercially managed solutions is because they don't have local expertise or staff to do things themselves. I do agree that on-prem solutions are better, but Zoneminder is probably not a great option. Besides being old and clunky, it also isn't anywhere near a complete solution, and the IP cameras people often choose to connect to them are often security nightmares. There are many good and complete commercial offerings that are secure and keep video locally.
That and paying to offload legal liability to a vendor.
Lots of great, free, widely adopted open source technology solutions aren't adopted by public sector because their legal staff won't accept the liability of not having a paid contract that makes guarantees. Great use of tax dollars.
I totally get what you are saying and there are certainly some schools that lack IT staff, budget and experience but there are some schools that have big budgets and plenty of IT people sitting on their hands that could slowly build this out, document it in a way that schools could budget around YoY and set examples for other schools. Maybe even use it as a project to get students some college credits.
If there are better options than Zoneminder please do share the tutorial videos with others here so they have greater options. I am old and clunky so ZM works for me. Some may even say old and clunky can mean reliable and low maintenance. There are probably some school IT admins reading this. ZM has great documentation and tutorial videos in my opinion. It is also used by a large number of corporations.
Just my own philosophy but I am leery of expensive turn-key commercial solutions as they lead to proprietary solutions that school IT won't understand and will just lead to dead cameras and empty NVR's when law enforcement need them the most. It will be one of the first maintenance contracts that get cut from budgets.