Coming this August - Locked mode in Quizzes in Google Forms!

Google Forms Updates & Improvements
Locked mode in Quizzes in Google Forms will be available starting in August. In addition to Locked mode, Google Forms will now enable you to import questions from other Forms into new ones in order to save time.

From the article..."Educators can enable locked mode in Quizzes in Google Forms on managed Chromebooks. This mode prevents students from navigating away from their assessments until they submit their answers, which helps them focus during quizzes and encourages academic integrity. Thousands of educators used locked mode in beta, and this August locked mode will be available to all G Suite for Education users on managed Chromebooks..."