Being picked on in school its making me feel sad, dunno what to do really.?

Im a boy in year 9 and all this year and some of last , im getting picked on by this group of girls , they just always make fun of me and calling me names and just being nasty and making me feel very sad. Last week i was sitting on the bench , coz we had swimmin in pe and one of them said "hope u left some water in there fatty " and just go off laughing . They always make fun of me , i never react just bottle it all up i suppose , guess ill just have to put up with it , i cant really tell on girls anyway .