In my experience, departments can be re-structured, staff get replaced, budgets get changed, buildings get remodelled, torn down, or re-purposed. Frankly, if you expect such a project to survive even 50 years you may have to do a bit of planning first.
Figure out who is going to manage the whole thing; a system can't just be put in a closet in a classroom; find a central location (say, a large airtight, waterproof safe in the school library, labelled with a plaque, and get the school board, school paper, etc. informed about the project so that its existence is recorded in various ways. I'm sure that's just about the best you could do with your budget.
I'd also not recommend preserving just one system, but probably several complete ones, maybe of varying age. If you got a couple of 286's with PC-DOS, a couple of Pentium II's with Windows 95, a couple of original iMacs with Mac OS 9, etc, that might be much more interesting than just one system, and surely it's better to have some redundancy in case one or more of the machines don't survive for some reason. And certainly include as much physical media with as wide of a variety of software as you can...floppy disks, CDs, DVDs, hard drives, zip disks, and perhaps best of all would be USB flash drives as these would be more likely to survive than optical or magnetic media, and unlike these, USB mass storage might be possible to read with computers with computers built in 2020 or even later.
Miscellaneous tips:
I wouldn't bother with any software that requires online activation, active internet connection, etc. I'm sure the internet will be quite different from how it is today, and even software giants like Adobe or Microsoft may be long forgotten in 2060.
Make sure the systems POST without their clock batteries; these will surely be dead in 2060.
Include as much paper documentation as you can. Manuals, quickstart guides, printed tutorials, anything. The documentation on this stuff might be very well preserved online in 2060. Or it might not.