Extension: My extension for this activity is to have students add a background to their story and see if they can add movement to at least one of the sprites. For instance, they could make the teller of the joke look like he or she is laughing. This would keep the students who finish quickly busy, so I could help the slower students. Here is a link to my completed activity with the extension: https://scratch.mit.edu/projects/156832835/
Advice: Some of my students had a hard time understanding the concept of wait blocks - maybe when discussing polite conversation, have a few students role play a simple conversation and have the other students time them while talking/waiting. This may help them process the concept a little more.
Lesson Structure: I had my students work on a dialogue in small groups. I told them to keep it school appropriate and be at least 5 lines. We then discussed what polite conversation looks like (do we interrupt? talk at the same time?, etc). I had a few groups act out their dialogue. I had them open Scratch and start a new project called joke. We went through the rubric and then I showed them the “say____for___secs” block and the “wait” block. I got them started on the assignment. I changed the rubric a little, here is a png file: 
Additional Resources: I would maybe have a few links to clean knock-knock jokes for the students to look at - not very many of mine could come up with their own.