At the end of an academic year or term, you can archive your classes. This will clear up your My Classes page, making it easier to work with your current classes. This will also return the student subscriptions used in that class to your currently available pool of student seats (until the seats expire).
Click the three dot icon on the right of the title bar for the class you want to archive. Select and click Archive Class
Here's an example of how archiving a class affects your pool of available student seats. Say you purchased 120 seats for a school year, and created four classes, each of 30 students. In the next year, you renew your subscription, this time purchasing 150 seats. When you purchase your new subscription, your pool of available seats will increase by 30, since you purchased 30 more seats than last year. To access all 150 seats, you need to archive last year's four classes to return those seats into your pool of available seats.