You can now set up a section as an optional class. Teachers see them as normal, but students don't see their periods by default. Instead, they see a green triangle in the corner of the period, which they can select to open the section picker.
In the example above, the section was set up to auto-enroll all students from grade 5, so that only those students can see the green triangle and pick an occurrence.
Optional settings can be found at the bottom of the section details, when selecting "Edit details" on a section. Normal sections will show this:
You can change that mode to either "by occurrence" or "by semester".
Once a mode is selected, you can:
Set the number of days before each occurrence a student can select that class
Set a maximum number of students who can select each occurrence
For the semester mode, students selecting any occurrence in a term will automatically get enrolled in all occurrences of that term.