Forgive me if this has already been said, but people tend to dislike the class under them. No idea why, and sometimes I'm even guilty. I remember back in seventh grade, other kids in my grade were like "dude loving sixth graders are SO ANNOYING", and then the very next year they said the same thing about seventh graders.
It's just an ongoing superiority complex naturally present in a lot of people because they've been given one reason why they're better than the lower classmen: they're older. It's not a legitimate reason to forget with innocent people but they tend to anyway because hell, forgeted if I know why people do what they do.