If you can get by with Linux/OpenBSD/FreeBSD, PXE boot is the way to go; with a read-only network share for applications, and a read-write one for user files. Greatly reduces TCO--only one HD, only one machine to maintain, and any virus intrusions on clients can be cleared with a reboot.
I work at Hack Club (https://hackclub.com), a nonprofit that supports teenagers in finding and pursuing their love of computers through free programs.
First, this is amazing! Thank you for helping make this happen.
There are a number of Hack Clubs that run out of computer labs like these. My #1 recommendation would be to have a concrete plan in place to ensure high quality, reliable internet access, and the ability for students to access the space recreationally.
Many of the Hack Clubs we have in underresourced areas that run out of labs like this don't have access (or lost access) to the internet, and for many of them we will reimburse the cost of a 4G phone hotspot. But it is not a very good solution
The other challenge a lot of our teenagers run into is the schools only allowing them to use the computer lab during class hours. I think it is very important that students are able to come to the lab outside of normal class hours to experiment, play games, etc - as it's difficult to find a love for computers if you don't spend a lot of time around them.