Students can create their own accounts using Google Classroom and have those accounts automatically tied to your teacher account.
First, make sure there is a class on your Roster page where those students belong. If you need to create new classes, here's how.
Next, click Add Students on the Roster page and choose the Class Code tab (if you do not see that tab, see Class Codes):
Copy the link to the right of the correct class and give that link to the students (or you can give them just the Class Code and they can go to www.whoosreading.org, click Log In, click Have a Class Code?, and input the code).
With that link or code, they will be able to sign up with Google (they do not need to fill out the boxes, just simply click the Sign up with Google button at the bottom):
Students will be prompted to log into their Google accounts if they aren't already, or select the account if they are already logged into Google.
They will then be taken to their newly-created Whooo's Reading student account, which will be automatically tied to the class in your teacher account that is associated with that Class Code.
To log into their accounts in the future, they can do so with Google, just like the rest of your students (they will not need the Class Code any longer). See how students log in with Google.