Reports & Discoverer :: Adding Sequence To Report

I'm having a report and I want to add sequence. My report is group by type. I want the sequence to restart for every new group. Lets say a report having name of students of different classes and grouped by class the result should look like this:

class A
1.student1
2. student2

class B
1.student3
2.student4

what I did is created a summary column with count function with source student but the sequence did not restart when group is changed .