This is a little chunky, but here goes. Use the LARGE() function,
which allows you to pick out the Nth largest number in an array. So if
your 20 scores are in A1:A20, it would look like this:
=AVERAGE(LARGE(A1:A20,1)+LARGE(A1:A20,2)+LARGE(A1: A20,3)+
....LARGE(A1:A20,10))