Several times I’ve found cash I forgot I stuffed into my bag earlier. Always a pleasant surprise.

A brick.

No, I won’t explain.

I (still) don’t have any friends, so on 1st year of high school I tried to be as interesting as possible. I kept bringing some electronics, but most interesting was fluorescent light bulb and Tesla Coil. OK, OK, it’s not a Tesla Coil but a slayer exciter circuit, but it’s close enough.
Other stuff I can remember is: USB buck/boost converter (basically a low power variable power supply), USB alligator clips, USB tester, smart load tester (FX-35), battery charge tester, hand crank radio/flashlight combo, low power FM radio transmitter, bootleg NES game console with composite video output, blank DVD, universal TV remote, 5 phones at once and other stuff.
Although, you could say the most unexpected was the HP PSC 1315 printer. One teacher told me I can take it home, and it fit precisely into my backpack.

Nowdays, 2 years later, the best you could find is RTL-SDR and the extendable dipole antenna.

A dead rat. I found it, gave it funeral rites, put it in a tiny plastic coffin, stored it in the freezer. From time to time, I’d take the coffin out, put it in my bag, carry around with me for several hours, so the rat’s soul wouldn’t feel lonely. Then back in the freezer. This went on for at least a year.

Unexpected for ourselves or for the rest of the classroom/teachers?

Not a lot I can think of but I do remember with horror when a classmate opened his backpack an this cockroach just came out of it running for its dear life

In MY bag? Nothing really comes to mind. In someone elses bag tho?

It’s been years and I still remember the guy that carried around a gym bag with wheels of cheese in em in my community college. Dunno if he was selling, but i was always kinda wanted one NGL