Why are people such jerks in 5th Grade?

Hi! I'm a guy in 5th grade and I've noticed that a lot of people are jerks to me and other people. In 4th Grade people were nice and respected each other. I'm a little plump and in 4th grade no one made mean jokes about other people's weight or other things like that. Now people are like, "Andrew, I'm amazed you kicked the ball in kickball," and, "You know Andrew, you sacrifice athletic abilities for brains." Why are people such jerks?!?! They make fun of each other and are mean! This one kid, Peter, liked this girl, Julia, but so did Austin. On valentines day Peter got julia some chocolate and a necklace. Austin got her chocolate. BAM! Next day Austin is at School with a necklace! They hate each other and Austin is a jerk to peter and made him cry several times! What is wrong with people?!?! Can somebody please tell me why people are like this in 5th Grade??? Thanks

Why are people such jerks in 5th Grade?
They are immature, you are mature for you age, I know I was and it was kind of hard for me to deal with this childnish. I got along okay by just ignoring it, they'll grow up some time, you guys are still young.