One of the challenges of accessing classroom from a mobile devices is that there may be multiple Google accounts already signed into the device. Your student's access is tied back to her or his @d93mail.com account and it's important that the account you are accessing classroom from belongs to your student. To make sure you are using the right account follow these steps.
Open the Classroom App
You may now screen that looks like this. The first thing you want to do is make sure the right account is selected by pressing the profile icon.
A box will pop up allowing you to pick which account you would like to sign into. If you see your student's @d93mail.com account select it. If you don't see your student's @d93mail.com click Add another account
Sign in with your student's account.
You should now see your student's classrooms. Your student may have one or many depending on grade level.