Choosing the right OS to match the industry is an impossible task. But I'll suggest a few avenues you might want to explore.
First you want to know which industry. If your scope is wide (as I expect it would with students and teachers using 5000 computers), then the industry range is wide too - which muddies the waters some.
I currently work for a bank, I have worked on defence, communications, air traffic control, embedded systems, both as a supplier and as a client - so quite a range of industries.
I found that large institutions try to use as few platforms as possible, which means that they only upgrade their systems as a whole somewhere between once every 3 to 6 years or so - add to that that generally corporations do not buy the latest out on the market, they buy a proven product. So while one corporation's systems might all be more or less the same for desktop applications they might lag by up to 10 years behind the leading edge if you're at the tail end of the update cycle. Or they might be reasonably up to date.
On the other end of the spectrum are small organisations. Quite a few of those will not have a standard policy and might use a multitude of systems.
So by and large for desktop use (office applications - which everybody needs in their daily work), but alos frequently for development, I believe the most common are variants on the Microsoft theme ranging from the oldest possible to the newest possible.
I am not very connected with any industry that puts the Apple solutions forward, so I cannot comment on this.
But for production and development systems I have seen and used a whole range and mix of Windows, Linux, proprietary Unix, and other older beasts (mainframes and similar dinosaurs).
So personally, what I would suggest is to equip your teachers and students with basic computers (choose one platform only, the one that is the easiest for the IT support group to work with), pick a cheap platform that you can easily multiply and scale to as many users as possible and for specialist applications use large server boxes to host virtual servers to access remotely. My personal choice would be for a cheap multi-screen intelligent terminal based on some Linux solution with VNC, X-Windows, or all sorts of "remote desktop" type applications, with a few large servers to offer a variety of Windows (95, 98, NT, XP, XP Pro, 2000, Vista, 7) and a variety of Linux systems (I don't know how to virtualise Apple solutions, I'll let you look that up). That way you have the best of both worlds (with a few snags). A large but simple network of computers to administer with basic and standard functionality. And a few tricky virtual servers that require specialised expertise, but being virtual they can a lot more easily be backed-up, a snap-shot taken, restored to a previous state... Plus you can probably hire expertise as and when required for those once in while situations when one of those systems you don't know so much about gives trouble.
Only my two cents,
w.