Google Forms Issues in Google Classroom
Here is a workaround to use when your students are not able to click on Google Forms that you have attached to assignments in Google Classroom.
From your Google Form - Click the purple SHARE button and COPY the link to your form.
In Google Classroom - Edit your current assignment (where the form is not working) or create a new assignment and PASTE the link to your form inside the instructions/description area of the assignment (not where you would typically paste it to attach the form).
Click SAVE
Click this link for a quick video tutorial walking you through the process. Read more about this issue in the Google Support forum.