Explanation
You can use the RANK function to rank numeric values.
RANK has two modes of operation: ranking values where the largest value is #1 (order = 0), and ranking values where the lowest value is #1 (order = 1).
In this case, we are ranking test scores, so the highest value should rank #1, so we omit the order argument, which defaults to zero:
=RANK(C6,scores)
The following formula, which includes order set to zero, is equivalent:
=RANK(C6,scores,0)