Si sacas un 13 se te hace la media
 

How can we translate this :
Si sacas un 13 se te hace la media con las otras notas siempre que saques más de 17 en las otras partes.

If you get 13 marks in one part of the exam, you ´ll be have the average made in the final mark provided that you get 17 marks in the other parts of the exam.

If you get a C on the first part of your exam, you still have a chance of an excellent grade provided that you get A's on the subsequent parts.

Se os hace la media, como se dice?

it is averaged (in)

Poli and Rusty are both right.

I'll take a stab at it:

Si sacas un 13 se te hace la media con las otras notas siempre que saques más de 17 en las otras partes.
If you get at least 13 points, it can be averaged with your other scores as long as you get at least 17 points on the other parts.