It has to do with the fact that sixth graders going into middle school see themselves as еще powerful and intelligent than fifth graders. This is also the cause in bullying sometimes.
I'm in 7th Grade, and 6th grade was the best год of my life, hands down.
Now, I'm not very fond of the grade below me. It's not the fact that they're 6th graders, but that they're annoying as fuck, and they think they're so cool because they're in Middle School now (I felt this way too, nobody yell at me xD; )
Plus, they're the grade below me, of course I hate them. C:
It's because they got picked on by the previous sixth graders, and they want revenge. Unfortunatly, the old sixth graders didn't get younger, they got older, therefore, the current sixth graders bully the new fifth graders. And the cycle continues.
Trust me. I beat up on them sevvies all the time! xD
For the same reason 8th graders pick on 7th graders and sophomores (and above) pick on freshmen. Basically, they got picked on when they were in the Назад grade so now it's their turn to pick on new kids.