Oh, look Mum, the "RISC OS needs no maintenance and Windows is crap" debate again.
The reason, in my experience, that schools migrating to Windows systems from RISC OS experience a massive increase in support costs is because they buy the lie that all PCs are equal. Cheap computers installed by muppets will cause problems, shock horror.
Buying decent kit, having it properly installed and not listening too closely to the LEA are generally good rules of thumb.
If you want a bastion of RISC OS, I can offer you Tiverton High School in Devon. State sector, 120+ RISC OS machines.
For a PC installation with longevity, have a gander at
where a suite of 32 PCs we installed 7 years ago has only just been replaced.
As a supplier, we operate on the principle of not stitching up customers. Which might be why we're still doing the business after a decade, unlike so many in the sector. So the schools with whom we deal tend to not need loads of support time, or full time technicians.