Why do my clmates make fun that soccer is easy?

Absolutely not! I've been in your position before - it's jut that the other girls are secretly jealous of you! Obviously playing soccer is WAY harder than cross country! Not only do you run countless numbers of miles during every game, and even more at practices, but you have other things and skills to focus on besides just the running. Talk to your coach and see if they'll let you set up a day where you can have these girls come to practice with you. Then they'll see just how much effort, dedication, and hard work soccer really takes! It'll shut them up for good!