I know it seems kind of retarded. But a school does that because it makes their jobs easier, the same reasons most companies do it to. They may be using applications that don't fully support new operating systems, or their technicians have systems in place that make changing to a new OS hard. Plus there's a cost to transitioning to a new OS, when you can just image every machine with what you've had and used for ages.
Eventually they do upgrade, but as my college proved for the first year of Windows 7 use, it really does fuck with how various things work. And the technical support staff had more problems than ever for the first few months while they found out what does and doesn't work during actual usage.
I know this is a late reply, but I never understood the reasoning behind this. It might be hard to upgrade now and deal with the problems it creates, but putting it off just makes those problems even worse.