At DD's school. The (Scottish equivalent of) the Y12s are not required to be at registration, but their attendance at timetabled classes is tracked. They must sign in and out of school if they miss registration or leave early.
Everyone who enters the building must either be registered or must sign in. It's a safety measure so that everyone can be accounted for in the event of a fire/other disaster.
I think all large buildings where a large number of people go in and out frequently work this way. Legal requirement I think.
I wish I'd had the option of asking questions on MN when I was a teen. Could have been very helpful.