"Stop hurting yourself"

The sighn, "Stop hurting yourself" means to stop slacking off in class, and focus more on work. Last year when I was in the 8th grade, many people wouldn't listen to certain lessons that we were learning on our Science class. Then one afternoon, we were informed by our Science teacher that we were going to be filmed as a class doing a lab. On the day of the lab, many students had no idea with what they were working with, so the lab was INCOMPLETE. This situation affected my Science teacher, because our job was to impress the filmer, not emberass our teacher; when cleary she would teach. Basically what the quote is saying is that you are not only hurting yourself, but your fellow classmates by wasting your/our time and not learning.