When I was the IT manager of a not so small (750-800 pupil) school in Hampshire, I had to manage two distinct networks: the old RISC OS on, which was mainly 710 RiscPCs with the odd StrongARM one, and the new Windows NT/RM Connect based one. I was part time. I have to say, that the Windows-based one was a lot easier to maintain and keep ticking over. This could easily be seen by the amount of time I spent on each of the networks: The Windows network consisted of two suites of 30 machines, plus another machine in each other classroom, so approx. another 50. I spent perhaps 2 hours a day on it. The RISC OS network consisted on one suite of 15, and about 10 others scattered around. I spent about 4 hours a day on it.
I'd certainly never want to maintain a large RISC OS network ever again, especially not one with a RISC OS box as its file and print server.