You can archive students who have left the school, so you can focus on the newer cohorts. You will still be able to see all archived students and un-archive them if needed.
Archive students
Step 1: Navigate to Students in the left-side menu.
Step 2: Using filters and the checkboxes, select the students that you wish to archive.
Step 3: Click the Archive button.
Unarchive students
Step 1: Navigate to Students in the left-side menu, and click on Archived Students at the bottom left of the list.
Step 2: Select who you wish to unarchive.
Step 3: Click the Unarchive button.
Note: You can still click into an archived student to see their data.
While you're here, see this video for a quick overview of archiving students: