This applies to students, teachers and administrators.
You can log in without your school's login URL. To do this, just go to https://testy.me/login and search for your school using its name or its abbreviation. During this login process, once you are asked for your username and password, your school's login URL will be the current URL in your browser.
If you are not sure, you can always ask your teacher.
If your school uses a custom login, this may not work. In this case, you'll need to ask your teacher for help.