Inactive students (those students who have never been invited or registered their account) basically are just providing historical data in your high school. These students can be removed entirely from your high school, or you may keep their data as needed by just letting them remain inactive.
To remove inactive student account entirely, navigate to the student record by doing a search for the student in the search bar at the top right corner. Select the student from the drop down choice.
On their profile page, you will see a 'delete' button. Clicking this button will remove the student and dis-associate them with your high school.
The Student should be removed from your high school.