When you create a new class, you can now add a class end date. When a class reaches that date, all students are automatically removed from the class roster. This allows teachers and a class to remain in session and ready to enroll a new set of students at the end of a semester or term. 

If you did not set an end date, you will need to manually remove the old students from your class at the end of a semester or term and add new students. 

If you made a mistake and your students were removed, please visit this link How Do I Restore A Class If My Students Were Removed On The Class End Date to learn how to Restore your Class.