Crossovers That Should Not Be 19: Making no sense, right from the gecko

[drakensis]

Faced with the menace of Koro-sensei, the Earth's governments develop a weapon that can blast a limited volume out of this dimension entirely! Sure it's a little drastic but when you're faced with imminent world destruction, sometimes you need drastic. The weapon is deployed on the way back from Class 3-E's summer vacation since it's easier to cover things up if all that vanishes is one small boat in the middle of the ocean. The cover story practically writes itself.
Sure, it's a bit rough on the entire class of middle-schoolers on the boat but they'd die anyway if the Earth is destroyed and the School Chairman assures the governments that no one important is being lost and he's happy to donate the school's boat to the cause of getting rid of Class 3-E... of Koro-sensei.

All very well for that world, but where to Class 3-E wind up?

Lots of possibilities for a crossover, but what about... Naruto?

Sure, the kids aren't going to be chakra-users so they aren't going to be any match for Narutoverse ninja in combat, but they aren't being trained for combat. (and if combat is required, Karasuma is pretty good, not to mention Kuro-sensei is at least S-rank). They could very easily become a minor faction in their own right. Plus Kuro-sensei would probably delight in dressing as a Kage.

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There's also another story possibility.

Replace Koro-sensei with Piccolo from DBZ.

Evidently inhuman? Check
Practically invincible? Check
Destroyed the Moon? Check
Teacher? Check (well, taught Gohan).
D for Drakensis

You're only young once, but immaturity is forever.