No. Octopi is just wrong, has no justification, and is based on ignorance.

Tell my 2nd grade teacher.

No. You seem very confident, but octopi is (grammatically) perfectly acceptable. The English word octopus is taken from the Latin (coined by Linnaeus) word octopus, which is of Greek origin. Greek loanwords in Latin were used very inconsistently, but many people on the internet seem to believe that it MUST be a third declension noun. However, Pliny treats the structurally identical polypus as second declension and uses the plural polypi (see https://www.perseus.tufts.edu/hopper/text?doc=Plin.+Nat.+9&f...).

> no one cares

My elementary school teachers sure did. And they insisted that it was "octopi".

Excuse me if this may sounds harsh or uncharitable, but my experience with American elementary school teachers has led me to believe that many, if not most, elementary school teachers are generally rigid in their way of teaching (my way is the right way, there is no other way), and refuse to address nuance, perhaps from their own ignorance, their patronization of children, or both. While we absolutely need and should support high quality elementary school teachers, the assumption I've developed is that most elementary school teachers are highly juvenile themselves, and are there partially due to their own lack of executive functioning, considering that elementary school is more similar to a day care than an actual educational facility.

I recall talking to an elementary school teacher who insisted that the only way to comprehend whether a number was even was to determine if the least significant digit was 0 or 2 and I was met with hostility when I mentioned that while that may be easier for the child to understand within a base-10 world, if even the educator can't understand why an even number is more accurately described as a number which when divided by 2 has a remainder of 0 then I should be very concerned about the state of education.

It’s possible. I suspect it may partly be that stories about elementary teachers being stubbornly wrong are more interesting than stories of them being stubbornly right or being open minded.

I can agree that I'm most likely experiencing selection bias here.

I haven't heard about much besides perhaps even stronger rigidity from other places so we might not be especially bad and I've only heard of softening in the US lately.

But spending too much time on nuance may be especially counterproductive with younger children. I'm not sure about that but it's a reasonable hypothesis.

To boldly go.

Octopux.

Its Engl-ish. Emphasis on the -ish.

They’re all fine.

I think you are correct on the first point, but judging by the other comments here (and any time this subject crops up), you are apparently wrong on the second. :P