Can a professional teach classes without panel access?

Yes. Leave their panel permissions off and keep Show in calendars and classes on.

I have classes that don't need a professional (for example, open gym). What do I do?

Every class needs a professional. Create a generic one (for example, "Open gym") with no panel access and hidden from the app.

Is a professional also a client?

Professionals have a client file too. Tick Exclude from reports in their profile so they don't count in your figures.

Someone on my team can't see a section. Why?

They're missing that section's permission. Check their panel permissions.