Say your peers scores are x1, x2,..., x{n-1}, and yours is a.
Then you want to show that a is greater than the average, written as an inequality you get:

(x1 + x2 + ... + x{n-1} + a) / n < a

Where n is the total number of grades including yours.

Now multiply both sides by n to get

x1 + x2 + ... + x{n-1} +a < n * a

Subtract a on both sides

x1 + x2 + ... + x{n-1} < (n - 1) * a

Now divide by (n - 1)

(x1 + x2 + ... + x{n-1}) / (n - 1) < a

The LHS is the average of the scores without taking into acount your score.

Hope this helps :-)