I'm lucky to be part of the GSuite Experts Team who respond to support requests on the Google for Education Help Forum- I love helping out teachers when they're stuck. Many teachers have asked for a way to keep absent students from accessing quizzes in Google Forms until they're back at school. Others want to post the quiz, but not have students access it until they're ready to take it.
In these instances, I always recommend teachers use the "data validation" feature in Google Forms to create a sort of "password" for your quiz. The video below outlines the process for you:
If you try this out in your classroom, I'd love to hear how it goes in the comments!