A 20 lb (or so) sealed lead acid battery and an inverter, at U Nebraska at Omaha around 2003-2004. I had imported a Sharp SL-C700 and it was very power hungry. Smart phones were barely a thing (blackberries) at the time.

I think I was vaguely aware of the possibility of some unexpected metal shorting the battery and getting hot enough to start fires, so I bought a green rubber bath mat (which I remember had little sucker feet on one side) and wrapped it around the battery.

I finished my undergrad in 2004 with no incidents.

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 bottle of mustard. I add it to my hot dog during lunch.

A brick.

No, I won’t explain.

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

A large green pushbutton with a Molly guard labeled “OK” I found in a scrap pile behind the physics building.

A boa constrictor. Pulled it out in english class and suddenly I was popular for a few minutes

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.

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

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

I try to keep it predictable (for a lack of a better word), but probably a supermagnet, like the kind you might get from the insides of an old computer. Helps adjust (or destroy) anything hands cannot.

Cockroaches

A built Lego set, specifically the 40567 Forest Hideout.

It was not unexpected thing for me though, but for everyone else.

set of screwdriver, a bag of PC screws and a battery operated soldering iron with some solder

Packets and packets of Tapatio. I would steal a shit ton during lunch in HS. I had so much I had two trash bags full of them