By the way, I can guarantee you that there is at least one other parent in the school who enters *without signing in* (a spouse of a school employee, a parent *liked*/favored by the school administration.
All it takes is for witnesses to verify that Jane Doe/John Doe was in the building and to check the sign in log to see if he/she signed in to prove.
I’ve been “waved on” when I’ve come to the office before. But I did go to the office to sign in and I was given permission to “go on” from the office without signing in (that was one time). But note ... the purpose of going to the office is “getting permission” to proceed past the office. That’s the key item.