Assassination Classroom: Extracurricular Lesson is an ONA with 8 episodes of 5 minutes that combined form 2 normal episodes of 20 minutes. The first normal episode (1-4) takes place between episode 1 and 2 of the second season of the main anime and the second normal episode (5-8) takes place in the middle of the 20th episode of the second season of the main anime. They're basically two extra episodes of comparable quality to the main anime. The story for the first episode was surprisingly good and actually had an interesting and unexpected twist. The second episode doesn't have much of a story but isjust some extra content.

The characters are the same as in the main anime however the focus in these two extra episodes is on some characters that had gotten a bit less attention so it has some nice character development.

The animation, sound and voice acting are all the same or on the same level as the main anime so it's of very similar quality.

Overall this ONA is nice to watch for just a bit of extra content. The quality is the same as the main anime and the character development adds to the main story. It's best to watch them in the correct context but watching them afterwards is fine too.