Go to Classes and select Create class. Give it a name (for example "3 Blue") and a year level. Once created, the class page shows its own class code - a short code you can write on the whiteboard. It isn't a secret; it exists purely so students can find the right class quickly when signing in - see How students sign in.
Deciding who can see it
On the class page, admins (and anyone with whole-school access) see a Who can see this class panel. This is how you control which staff members can open a given class at all:
Find a teacher and select Assign. By default they get full access - view, edit and delete.
Untick any of those individually if you want a teacher who can, say, view a class but not edit it.
Select several staff at once and use Apply to selected to set the same access for the whole group in one step.
A teacher who isn't assigned to a class simply doesn't see it - not on their class list, and not if they try to open its address directly.
Editing or archiving a class
Open a class and select Edit to change its name or year level. Archiving a class (when it's no longer active) lives inside that same Edit screen, behind its own confirmation - it's deliberately not a one-tap action on the class list, since undoing an accidental archive is unnecessary friction to add to a screen you open constantly. An archived class can be restored with a single tap from your class list.