Hello all,
Use the following workaround.
In the main quiz settings, assume that "Open the quiz" is set at 9:00 am and "Close the quiz" is set at 11:00 am, i.e. this is a two hour exam.
Create another quiz, let's call it an access quiz, containing a simple MCQ, T/F or SA question, like "Do you want to go to the main quiz? Y or N" or any other boggus question. Tell students that they must answer and "submit all and finish" in order to have access to the main quiz. In the settings of this access quiz, set "Open the quiz" to 9:00 and "Close the quiz" to 9:15, i.e. it is only open for the first 15 minutes.
In the main quiz settings and under "Restrict access", add a "Grade" restriction and select the access quiz. If you do not specify a grade, the restriction is satisfied regardless of the grade.
You can also add "Date" restrictions to both quizzes to control their display.
Thus, punctual students will have access to the access quiz from 9:00 a.m. to 9:15 a.m. and therefore to the main quiz throughout its duration. Latecomers will not have access to the access quiz after 9.15 a.m. and therefore they will not be able to access the main quiz at all.