Haha, because those future Walmart employees are feeling like Mr. Superior and decide to be brainless lunatics by making themselves feel better by bullying smaller people that they think are beneath them in the stereotypical social class. Yep, they should just watch and learn; when you become a successful person in life and they work at all of these lower-class places for the rest of their lives, who will be laughing then? (Sorry it is so harsh :( My friend took her own life because of people like that :(
Honestly, I don't know why people do that. Maybe it's because the upperclassmen feel the superiority over freshmen, so they use that power and make fun of you.
I'm an upperclassman myself and honestly, sometimes freshmen annoy me. I know I am older than them; therefore, I feel more superior. I don't have anything against freshmen who don't irk me. I usually have freshmen friends.
They usually do it to show who's the "boss", some students do it, some don't. I'm a sophomore and I've seen some friends and many students in other grades. They also do it for fun since they just came to high school and are new and small. So the other students would go "Welcome to high school" and make fun of them until they get tired or after freshman year ends.