To join a class, your child must log in to classroom.google.com with their school account and click on the + icon on the top right-hand side.
On the new page that appears, they should enter their class code into the box and click the join button, which will turn blue when a correct class has been entered.
The class will now appear on the page at classroom.google.com. They will not need to enter this code or join this class again.
Open the Google Classroom app, log in and click on the + icon in the corner of the screen. A new screen will pop up asking for a class code. If a correct class code is entered, the Join button will become available to click and confirm.