Ansatsu Kyoushitsu: Deai no Jikan

One of the best specials i've seen (i didn't see much, though). The other one would be Steins;Gate one.
It is "kinda prequel, kinda not"

It's interesting, it's fun, it's compact. It also has good music, like an original show. Yes, it doesn't add much to the series, but then, how many specials do?

It's actually not that bad to watch on its own, without series, because of how story is served

I absolutely recommend this concentrated fun package. It won't take a lot of your time anyway. Also, you can use it as the show ad to agitate friends! If that is even possible

...I don't know what ... else to type without spoilers about non-serious 10-minute show, but this site forces me to write a longer review, so be it, i am making it longer, just to make system happy and everyone reading this too, so long, just wow...

Assassination Classroom: Meeting Time is a cute 10 minute episode about how Koro-sensei met Karasuma that you can watch at any point before, during or after the main anime for a bit of extra content.

The story is very simple and it just concerns a first meeting between Koro-sensei and Karasuma, who are therefore the two main characters in this special. Their personalities are neat and reflective of the main anime.

The animation is quite good and it has the same animation style/quality as the main anime. The voice acting and sound are also good and the voice actors are all the same as in ... the main anime as well.

Overall this is a fun small special to watch for a bit of extra content.

Reviewing a special... I've always wanted to do this. XD

So this episode is a little 10 minute extra to the main series which chronicles the first encounter between Karasuma and Korosensei, as told from Korosensei's point of view.

This is pretty much comic relief for followers of this series since season 1 ended in a somewhat abrupt manner. It starts off on a serious note with Karasuma locking horns with Korosensei for the first time in a dank jail cell as the slimy octopus states his various demands and what not, and then the episode ends in a somewhat unexpected manner. The art and soundtrack ... are all consistent with that from the main series.

If you are wondering whether this episode is important, then wonder no more: it does not contribute to the plot in any way, so you may choose not to watch it. There are no teasers, no foreshadowing whatsoever. However... if you have seen season 1, then I do recommend you watch this just for the sake of closure.