The p5js.org website is a great resource. The basic documentation gives examples of all of the built-in functions: https://p5js.org/reference/
The examples section shows in-code examples of many cool concepts: https://p5js.org/examples/
The "learn" section provides some tutorials, but they still have a way to go: https://p5js.org/learn/
Daniel Shiffman's youtube channel, "The Coding Train" contains a large number of videos for both basic and advanced coding with p5.js: https://www.youtube.com/user/shiffman/ The p5.js playlist covers the framework we've been working with: https://www.youtube.com/user/shiffman/playlists?view=50&shelf_id=14&sort=dd
The creator of p5.js, Lauren McCarthy, has a introductory book (but I haven't reviewed it): https://www.amazon.com/Getting-Started-p5-js-Interactive-JavaScript/dp/1457186772/
Here's a free online course which looks quite promising: https://www.kadenze.com/courses/introduction-to-programming-for-the-visual-arts-with-p5-js/info
Openprocessing.org is a place where anyone can upload a p5.js program (also called a "sketch" on the site), and others can run it, edit it, and save their own "fork" (version) of the program to their profile. The site also makes it relatively easy to collect programs for teaching purposes.
Here's my profile: https://www.openprocessing.org/user/53170
And my 4-hours-of-code class page: https://www.openprocessing.org/class/56462/ (There's a tutorial series of programs here under the "Fundamentals" section, but the order is wonky - they start at 1, and go to 13).
Some other neat class collections I found:
The "Basics" section here includes some cool sketches, and the Audio section has some examples of audio processing (which is a more advanced topic): https://www.openprocessing.org/class/56617
Just some fun examples and basic shape drawing: https://www.openprocessing.org/class/56180
A few examples of simple video games, and including images in programs: https://www.openprocessing.org/class/56530
Really beautiful artwork from simple programs: https://www.openprocessing.org/class/55749
Various simple examples broken out by programming topic: https://www.openprocessing.org/class/57089
More examples broken out by programming topic, with an artistic flair: https://www.openprocessing.org/class/57353
More examples broken out by topic: https://www.openprocessing.org/class/56587
This one uses the original Java syntax, but the sketches here are just too cool not to post: https://www.openprocessing.org/class/25487
https://www.openprocessing.org/sketch/437061
https://www.openprocessing.org/sketch/416296
https://www.openprocessing.org/sketch/416164
https://www.openprocessing.org/sketch/426434
https://www.openprocessing.org/sketch/456295 (really sophisticated)
https://www.openprocessing.org/sketch/425701
https://www.openprocessing.org/sketch/419330
Sand game: https://www.openprocessing.org/sketch/441885
Flocking: https://www.openprocessing.org/sketch/445976
Fancy 3D demo: https://www.openprocessing.org/sketch/493904