To keep control of your assessments you can add a password to your form to make your students enter a specific number or word before continue. To create a password protected form follow these steps.
Click the New Section button on the right tool bar to get two sections to your form.
In the first section, add a new questions. Make sure this question is a short answer type of question. To prevent students from skipping the password, toggle the required switch on the bottom to make sure they can't skip the question.
Select Response Validation from the question menu in the lower right side of the question. You can select a word or number as your password.
For a word to be your password select Text from the first dropdown menu, then from the second select Contains. The next blank line will allow you enter the word or phase that will be your password. The last line is the error message to give when the wrong password is entered.
If you use a number as your password, select from the first dropdown menu Number and Is Equal To for the second. The next line will be where you can enter your number. The final line again is your error message.
Once you have the first page setup then on the 2nd Section, create your assessment as normal. You can make the remaining assessment a quiz or a survey, but to access it students must go through the first password page first.
To see an example, check out a password protected form here. The password is Photon.