First, make sure to include your students’ Google account email addresses when they are added to TypingClub.
Next, go to the Settings tab on the left-hand side of the page and click “Account Options”. You will see the header “Login Options”. Here, you can choose how your students can log in to your school's portal. If you check the "Enable option to login using Google", your students can only sign in with their Google account. This means your student must first log in to their Google account, then navigate to your school’s portal, and finally click “Sign in using your Google account”.