The classroom team has recently integrated Google Forms much easier while using Google Classroom.
Before this update, teachers would have to create an assignment and attach the Google Form link to the assignment using the Drive or link options. The students needed to open the assignment, click on the Google Form link to complete the form (quiz, survey, etc), and hit the submit button on the bottom of the Google Form. Then, back in Google Classroom, they needed to mark the assignment as done. It was a two step process to complete the assignment correctly.
Now, with the latest update from Google, the integration of Classroom and Forms is much easier. Please watch this short video on the new workflow of integration on Google Forms and Google Classroom.
I hope you find the screen cast helpful!