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 :-)

I was doing it the same way, but I thought it should be better and managed to shorten it even more.

The library is very nice but it's useless if you don't understand the exercise. How do you calculate the mean if you don't include yourself? Are you a student from another university?

Teacher, give me a zero, it's really bad. How do you calculate the average if you don't include yourself?