df['rank'] = df.groupby(['class'])['score'].rank(method='dense').astype(int)
stu_id class name score rank
0 1 A Jack 45 1
1 2 A Oscar 75 4
2 3 B Emile 60 1
3 4 B Sophie 64 2
4 5 B Jim 85 4
5 6 A Thomas 55 2
6 7 A David 60 3
7 8 B Lee 60 1
8 9 B Elvis 70 3
9 10 A Frank 75 4
10 11 A James 90 5
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