Hi  Ellie , thanks for asking this question. It sounds like you may need to expire some classes to free up some seats. Here's some information on what this means, and how to do this. 

Your institution’s Turnitin license is valid up to a certain number of students. Once the student limit has been reached, no new students will be allowed to join your school’s account (preventing them from submitting papers). When the limit is reached unexpectedly, it is possible that students have created multiple accounts to enroll in the same class. Each student enrollment counts as a licensed Turnitin user.

To view the student count for your school, log into your instructor account and click on the name of your school or institution. The account status page will show the account’s student limit, and the number of active students.

Multiple enrollments by a single student happen when a student forgets his or her password or e-mail address and does not choose to use the password help link or contact the class instructor regarding the e-mail address and name used.  If the student makes the request of the Turnitin Help Desk, it is possible to merge multiple accounts into one, provided the student is able to prove that she or he is the actual account owner for the accounts.  When a student has enrolled multiple times to one class, and has submitted papers through each account, then this would be the best option.

Students in *expired* classes do not count against your institution’s student limit. If you do not expire outdated classes, students from a previous term still get counted.  Please expire any classes they are not currently in use. Instructions are as follows:

From the Instructor homepage, click on the”edit” icon for the class (looks like a gear).
Change the class end date to a date in the near future (eg. tomorrow). The class will expire at 12:00am on the end date. (The class end date must be set to a future date).
You can also check for and delete multiple enrollments (duplicate accounts) of the same student by following these steps:

1. Sign into your instructor accounts.
2. Click on the class name.
3. Click on the gray “students” tab located to the upper left.
4. Click where it says “student name” above the list of student names.
5. Look for doubles in the newly alphabetized list.
6. Look at the dates next to the multiple students’ names.
7. Drop all multiples of each student except the version with the latest date (drop by clicking on the icon of a garbage can to the far right of the names).
Note:  When a student account is dropped from a class, so are the submitted papers under that enrollment email address.

As soon as the week is out (or once everyone is registered) the instructor should “Edit” the class settings and change the class enrollment key to the class, which will prevent students from being able to create a second account. To be clear, a student can only create an account if he or she has both a class ID number and class enrollment key. If your instructors change the enrollment key, students can’t create new accounts. This prevents the same student from creating multiple accounts.
The instructor can always look at the student roster list to confirm with the student which email and name were used to enroll in the class.
Alternatively, instructors can add students to a class themselves so that students never need a class ID number and enrollment key.
Please contact your account administrator for assistance in identifying duplicate students within the account, or purchasing additional student licenses.

I hope this helps!