Any number of terms for a release from some obligation or another may apply depending on its formality and terms (e.g. license, permit, warrant) and on its physical representation, if any (e.g. card, slip, chit). For the situation you describe, the most generic term is arguably pass. As the Merriam-Webster Learners Dictionary puts it:
a card or ticket which shows that you are allowed to enter or leave a particular place or to ride a vehicle
- a one-day/weekend/season pass to the amusement park
- Each new student will be given a bus pass. [=a ticket that permits you to ride the bus]
- We won backstage passes [=cards that allow you to go behind the stage] for tonight's concert.
- (US) You have to get a hall pass [=a card that shows you have permission to be out of class during class time] from the teacher.
The formal term for being late for a work shift or school period, at least in AmE, is tardiness. A document which allows a student to enter the classroom after class has already begun is a tardy pass or tardy slip, though simply late pass is also common. But these are for children, and in my experience, in the present-day, such things would be unusual in the American workplace. Borrowing school terminology may therefore be seen as demeaning or disrespectful, in which case inventing a novel term may be preferable.