Once a student log in Admin.google.com, after she log out. All others cannot log in regular gmail.
Web error message "admin.google.com is for G Suite accounts only. Regular Gmail accounts cannot be used to sign in to admin.google.com."
I type gmail.com, mail.google.com ... etc, all will redirect to admin.google.com and all login will have error message that I don't have G Suite, can't log in. There is no help page show people how to go back to regular gmail account.