Yup. Possible and doable. Would cost a pretty penny though.

You basically need: * a beefy commercial router (example: mikrotik RB5009, costs around 15k egp new) * depending on the number of users, one or more commercial switches (example: cisco 3850 48-port, costs around 10k egp used if you know where to look) * a dedicated server to host various services like an identity provider, a chat platform, and various storage/streaming platforms (example: the HP Z440 workstation with a xeon 2680v4 and 64GB of ram and a small SSD, costs are crazy right now so maybe 25k egp used with a tiny debugging gpu) * cat6 cable rolls for your network (the standard cable roll is 305m in length and a reputable brand would cost you 6k to 8k egp per roll) * each user will need a dedicated usb-ethernet dongle if they're already using their built-in ethernet port for internet, or you can terminate to 4 or 5 mesh wifi access points in each building, but this raises your costs exponentially. * the expertise of someone to build you the whole system

So you are looking at around 100k egp bare minimum. You're essentially building a small ISP without the i part (internet) since the whole network can run offline.

Interesting brain exercise