how to set the rollnumber for student

hi friends

I am doing school management projects. in that i want to set roll number for student and allocate the student for section of a class. here my doubt is class may have more section.like
first class has three section.how can set roll number for students of a particular class.for example each section have the 40 student of strength. then three section have 120 student. so here which one need to set first? section or rollnumber for every student of class. suppose if i set roll number first mean section b roll number starts from the 41. suppose if set section first mean i need to set roll number from 01 to 40. which one is best?