I only wear black and grey, at least 99% of the time to work. I’m a school teacher and have reduced and simplified my wardrobe quite a bit.
Anyway, instead of grey pants, is it OK to just have straight black? Black sweater, black pants?
Here is the exact sweater. Notice the model in the picture is wearing black pants.
I say yes. I do occasionally in my professional workspace.
You will probably get the occasional Johnny Cash joke, but hey…
Isn’t that the whole point of black clothes, that black goes with everything?
Yes. Not all blacks match accurately, due to different fabrics or condition, but I would still deem such minor differences acceptable.
All black everything is certainly legit, but maybe it is more of an evening look? I cannot find anything authoritative.
Yes, for me, wearing black pants instead of grey would be “flair”, so to speak. I’m doing it tomorrow.
By the old people.
I’m guessing they kids at school will have a more contemporary comparison.
(Who, you ask? Beats me. I’m an old one, too).
Oh, and as to the OP?
Sure, go for it. Bonus points if you add a van dyke beard and claim to be your own “twin”.
I’m not that fun, but thanks. 
This. A matte black fabric on your slacks would look like a different black than a black knitted top. But if that doesn’t bother you, go for it.
Yes, but you also have to be perpetually unshaven, wear a beret, and have a lit Gauloises dangling from your lip at all times.
Stranger
Sounds like a good plan to me, you Beatnik!
Over here, you would once have got jokes about chocolates
But what is it that makes you think you shouldn’t wear this? It’s not indecent or outlandish. Who would care and why would their reaction matter?
I’m assuming your lessons are interesting and engaging enough for your pupils not to take any notice, of course.
I wear very similar things daily and have never had a single comment from students.
I’m currently wearing black shoes, pants, shirt and hat.
My unspoken contract with my students was: you don’t comment on what I wear, and I don’t comment on what you wear (as tempting that is sometimes).
I consider this a plus.
Hey, who you calling old!? (says the guy who turned 68 las month)
Another vote that black pants / sweater / shoes is perfectly fine. Plus, it comes in handy after dark if any impromptu ninja activities become necessary.
Only if it is a Tactleneck.
Stranger
Ninjaed!
Nothing wrong with “all black” for a man IMHO. Back when I was working (office environment) my preference was some contrast such as black shirt, taupe pants, black shoes, there’s nothing wrong with the “all black” look whatsoever.