If you go to student details icon in the dashboard, it will take you to 'Student Details' page , where you can see there is radio button called 'Former Students' select the former student option and search the students as shown in the screenshot.



If the user needs to search for all the former students or former students with some other extra criteria, like all former female students, go to Student Details - Advanced - Give the search criteria and give former as option at the bottom. 

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