It rather depends on what kind of scale you're operating, and what kind of budget is available over what period of time. If you're relying on a third party to provide the hardware then you'll probably get a SAN as that tends to be what people know how to specify and support, and £50,000 over, say, three years really isn't that much for IT infrastructure for what, as you point out, is generally quite a sizeable organisation. Schools still tend to be under-provided for actual storage volume available - the amount of images and video the average school can produce in a year is growing and can quite overwealm some school's storage facilities.
A school has, in a lot of cases, a bigger user base than a medium and large business and has critical data which needs to be securely maintained, backed up etc. If anything the work being done on a school network is more taxing on the infrastructure than in businesses when you think about all the video and photo editing, data being pushed around etc.