Before logging in with Google, the Google admin user will need to integrate the accounts. To do this, they should log in to Just2easy (using either a free trial, manual account, or SSO service such as LGfl), and follow the instructions in this article: https://support.j2e.com/a/solutions/articles/77000532781
Once this set up process has been completed, all users in the school can click 'Sign in with Google' and enter Google log in details.