It is needed to add own score as there is a possiblity that you get an empty list (you alone wrote the test). This would result in division by zero.

Yes, adding own score is necessary to avoid divide by zero as you may get an empty list as input.

What if you're alone in your class, for example everyone but you is sick? You get an empty list and divide by zero. That's why you need to include your score for calculating the average.