The p5.js editor (created by Sam Lavigne) is currently in development, for your assignments you are welcome to try out the alpha. You can help by posting feedback and bugs Support for Windows and Linux coming soon, along with more features. Download the current release.
If you prefer, you could also just use a text editor. Here are some options:
- Sublime — recommended, need a license or deal with pop ups
- TextWrangler — free
- brackets.io — free, dynamic code update, a little more confusing gui at first
With a text editor, you’ll want to run a local server (Python SimpleHTTPServer or Apache) to test your work.