If you want to prevent students from filling out a Google Form before you are ready, try this hack to add a password to the form.
I had the Google Form as a part B to my test so I wanted students to show me they finished the Part A first before I gave them the password.
It is also easy to change the password. I edited the password for each class section.
Add sections to your Google Form. Add a short answer question with response validation. Changing the passcode is as simple as editing the response validation on the short answer question.