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.

The spanish inquisition

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 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.

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

I had a stink bomb, I had been a bit of a knucklehead and was kicked out from one high school to another.

When I got to my new high school they forced me too join JROTC because "PE was suddenly full". They just thought I was trouble...

Anyways, I took a really foul stink bomb that comes in this like glass container that you have to throw on the ground to crack open and stink up an area.

(This was Bush era and I was very anti bush and influenced by 2pac, I didn't wanna prance around roleplaying in a white man's army youth camp)

Well I was really mad they were making me wear those dorky outfits and I didn't wanna wear it that day, so I popped multiple packs of stink bombs into the JROTC classroom. I had some spot on baseball ⚾ aim because I threw 3 of em, from a good distance and they hit the $ spot right on the JROTC instructors podium under the BUSH portrait he made us to the pledge of allegiance towards, the US flag was next to the portrait. I threw it right before the class bell, and went to the bathroom. When I came back, lol everyone had evacuated the classroom it was unbearablem I played stupid asking what happened. Lol my fuckin squad leader whatever TF his rank was, told me that someone threw some stink bombs into class and that it would take place outside.

I'm very proud of that moment looking back. Oh and it was unexpected because I bought them and put them into my backpack from a swap meet trip earlier that week and I forgot to remove them at home.

One of the dorks from my JROTC class is the neighborhood LAPD pig where I live now not far from that High school, he remembered me and how I was trouble when he came to tell me to keep it down last year, when I threw a party.

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

When I was in middle school my brother was pissed at me so he replaced every piece of paper in all of my folders with pages of manga he ripped out of my one punch man book

Toilet paper. Because that stuff at the school was horrible.

My first try at college was in art school. I had wires with wooden handles on each end. They looked like strangulation tools, but they were to cut blocks of clay with.

...I'm pretty sure someone, somewhere has used them to strangle someone with.

Large bottle of bleach.