If you only want a count, please try table

text <- "ID Age School Grade
1 10 1 98
2 10 2 97
3 10 1 92
4 11 1 90
5 11 1 80
6 11 2 70
7 10 1 80
8 10 1 79
9 11 2 70"

df <- read.table(textConnection(text),header=T)
table(df[,2:3])

If you want sort the data, try order.

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