Let me tell you something as someone who watched and enjoyed the TV series that aired in 2015: Assassination Classroom is a great series, that is wildly enjoyable. I would highly recommend watching the series (linked on the MAL profile). I would watch this special episode if you can't get enough of the series.
Okay so this was made before the series and it essentially is a shortened version of episodes 7 and 8. Personally, those are some of my favorite episodes in the series, so this special is appealing.Anyway, lets begin:
STORY: 7
Good old Assassination Classroom. This is a great arc that has good pacing and (although predictable) a decent story line. The series is very humorous, which actually provides most of the twists.
ART: 3
Actually this is the worst part about the series. The animation is very pathetic and has pretty much nothing working for it. Granted that reflects the budget. Despite how poorly made the episode is, the characters are still distinguishable and recognizable.
SOUND: 4
I'll just come out and say it: most of the voice actors are all wrong and the acting is pretty weak, especially Koro-sensei. The sound effects are fine, but not notable. The series lacks any noticeable sound track.
CHARACTER: 9
The cast of this anime is fantastic, not matter how you spin it. The anime doesn't do a very good job at helping the characters come to life in that the animation and voice action is bad, but the characters as a whole reflect the greatness seen in the anime. This arch is very nice and touches on most of the cast for at least a little bit. It is also great how side characters (like Red Eye?) get enough screen time to develop and become likable.
ENJOYMENT: 8
I actually liked watching this special. Seeing how the series started is interesting to me as a fan of both the anime and the manga. As I said before, this is one of my favorite arcs, and you can see how much it improved when comparing the 2015 TV show to this.
OVERALL: 7
Okay so I weighted some of the aspects more than others. my statement on this would be "A fun watch if you are a fan of the series!"