From a class's page, Write a topic for this class opens Topic Studio already scoped to that class - the age band is prefilled, and the finished topic is visible only to students in that class. Everything else about writing a topic works exactly as it does for a family - see Writing your first topic and From draft to published for the describe-and-review flow, which is identical here.
Prefer Topics on its own (with no class selected) to write something every class can use - a school-wide topic isn't tied to any one roll.
Assigning it
Back on the class page, the assign panel lets you:
Pick a published topic.
Pick a mode - quiz, spelling, or Learn.
Optionally lock the timer, so every student in the class runs it under the same conditions.
Publish the assignment.
This reorders that class's hub so the assigned work appears near the top for every student in it - it doesn't lock the rest of the catalogue away, unless a student's own progression setting is set to work through topics in a fixed order.
Narrowing the AU Curriculum Pathway
If your class's year level has AU Curriculum Pathway content, a Pick curriculum topics button lets you tick exactly which year levels or topics that class should see from it. Leaving everything ticked (the default) means unrestricted - your class sees its whole year level's worth of curriculum content, same as before you touched the picker at all.