OpenProcessing allows you to set your sketches private by making it visible only to the class, only to the professor or hide source code. This is a great way to create assignments with an example sketch for students to figure out how to code. Or, you can ask your students to hide the source code so they can not look at each other's code for that assignment.
You can find these options when you are saving your sketch (click Save button on a new sketch, or (i) > Edit button on an existing sketch).
Under Who Can See Your Sketch?:
My Classes options will only allow all the students and professors in your classes see the sketch.
My Professors will only allow the professors in your classes see the sketch.
Only Me option will make the sketch hidden to anyone.
Under Who Can See The Code?:
My Classes options will only allow all the students and professors in your classes see the sketch source code tab.
My Professors will only allow the professors in your classes see the sketch source code tab.
Only Me option will make the sketch source code hidden to anyone.
Note that, when source code is hidden, it is still sent to the browser in the background for your sketch output to be displayed (under the play tab), so it may be possible for an advanced user to access the source code by digging into the browser developer tools.