Unfortunately, I couldn't get my hands on my school's old P.E.T. 4032, so the oldest computer I own is a mid-end custom build from 2002.
However, I have a Commodore SR 9190 Scientific Calculator that is, at my best guess, from 1977. While not being able to do fractions, it can do metric-imperial conversions, something I know my current TI calculator can't do.
My old school had some interesting shit though, already mentioned was the P.E.T. 4032, but it also had a Macintosh 512K (unfortunately, the floppy drive was broken, and we didn't have any System disks), an Amiga 500 (unfortunately, we didn't have Kickstart or AmigaOS floppies either, and it was missing the power adapter), and at least 20 Amiga monitors, because my school used to run on Amiga. I find it kinda sad, though, that they used to run on Amiga, yet only have one left, and don't even have a power cable for it. Of the 3 old computers they had, we confirmed 2 turn on, the P.E.T. and the Mac, but we only really got to "use" the P.E.T., and that was still a very cool experience for me, being able to use a computer older than my teacher at the time.
Now that's awesome, I'm used to schools using old Acorn Archemedes gear. The A500 doesn't require a kickstart, just bung a disk in and watch it load.