Here are the steps on how to add a student to an inactive class:

1- You need to make the session is active again by changing the session end date and changing the status to active. I usually just change the end year so that it's easy to change it back later on. Be sure to hide that session from the website so people do not accidentally book it. 

2- Find the inactive class, click on edit to change the class end date to a date in the future, mark as active, and save. 

3- Book the student and charge the fee needed.

4- Go back to the session, change the session end date to the original date, mark as inactive and done!