Before any student can sign in, an admin sets up the school's sign-in link from Student sign-in (this is the same screen a family knows as "Kids' login" - same setup, school-flavoured wording). Choose your school's slug and turn sign-in on.
Three ways a student can get in
All three land on the exact same student hub - only the door differs, so pick whichever suits your classroom:
Whole-roster picker - your school's link on its own (/c/<your-slug>) lists every student on the account. Fine for a small centre, unwieldy for a large school.
Class-code picker - your link followed by a class's code narrows the picker to just that class's roll before asking for a PIN. This is the one worth writing on a whiteboard.
Sign-in cards (QR) - from a class's Sign-in cards page, issue (or rotate) a code per student and print their cards. Scanning a card signs that student straight in - no typing at all, which suits younger classes best. A rotated or revoked card stops working immediately, the same as changing a PIN.
Shared devices
If your classroom uses shared tablets or computers rather than one device per student, turn on shared device mode from the Student sign-in setup screen - it changes how a session behaves on a device that's handed from student to student rather than kept by one.