Residual Plots. What's going on.

#1
I'm running some analysis on grades in different subjects and seeing if being involved in this outside activity has an impact on grades. I am looking at residuals for a regression I ran and getting this skewed residual plot. Any ideas what's going on? View attachment 5666
 

Dason

Ambassador to the humans
#2
It's not "skewed". It shows lines which indicate that your response is discrete. Is there a specific issue you're worried about?