If you really want to have the kids 50 years from now be able to experience what computing is like today, maybe the best thing to do would be to just keep the computer and monitor enclosures. Then, keep a copy of the hard drive contents yourself, backing them up and maintaining them yourself, and in 50 years, go back to the school. You could install a display inside the monitor case, use computer innards of the time, and then use an emulator to basically simulate the whole experience.
It's not a perfect solution, and doesn't cover certain things like an authentic input device (mouse, keyboard), etc., but to me it would seem to be more important for them to experience a close simulation than for everything to be completely authentic.
It would mean keeping the files from corrupting over the years, etc., so more work for you, but it would take care of the hardware failure issues.