What data structure to use for ranking system which divides itself in groups?
Just save the ranking score for every student. Calculate their group when you displaying them.
I have a quiz app where students can take tests. There is  ranking based on every test. It's implemented with simple lists (Every new score is inserted into the list and then sorted (index+1 is the rank)). 
But I want to add another abstraction. ie. Suppose 1000 students took the test and my ranking was 890. But those 1000 students should automatically be divided into 10 groups ie. group 1 of ranking 1 to 99, group2 of ranking 100 to 199 and so on. So if my overall ranking is 890. I should be subscribed to group 9 with 90th rank in that group. 
How should this be implemented?