Another nice thing is, that the Pi can boot and work solely on a single micro-SD card. That makes it easy to have any number of install "disks" ready at hand, you don't have to set up the systems hard disk first.
Finally, the Pi gives you a nice set of IO-ports, you can immediately start with any hardware tinkering project.
There's a hell of a lot of ex office PCs that can run python for much cheaper, if not for free
The Raspberry Pi 400 is a pretty much usable desktop computer out of the box which you can buy as a complete set for 100€ new, including a nice book. It is also very nice to use as it is compact, very portable and doesn't need more than an USB power supply.
In comparison it is very easy to grab an used screen, as the screen doesn't need to be set up. In addition, the Pi works well with a current TV. Brings back the best of the home computer age in the 80ies.
There are plenty of places that would donate to a cause like this. Or to anyone willing to cart the stupid hardware away, regardless of the cause. It is only "free" if one considers their time - and somehow, the gasoline they buy - to be worthless. The hours spent soliciting offers, picking up hardware, performing admin and tech tasks on old hardware, recycling the hardware that doesn't work, rinse and repeat would better be spent preparing a curriculum for the children.
Seriously, how does the above process not sound like hell to you?
The pi400 kit is $100. Flash five cards and you are done. If you scavange anything it's monitors, there is a whole aisle old flatscreens at my local goodwill, you could find decent enough ones for under $20 apiece. Alternatively any local techish business probably has a closet of monitors they would happily give away just to get them out of the building.
If you're trying to help other people, something new in box is 100% the way to go. It's not worth the time of gathering and messing with dissimilar hardware.
Both valid strategies. A question of pull vs push.
I agree about the open source part though, Raspberry Pi is a good fit for teaching purposes.
There you have it