please don't take what others say to heart. you're still very young, you are still growing! your body is still adjusting to all the changes that come along with puberty so ofc some changes like weight gain are natural. and besides... overweight by whose standards? the bmi? fuck that - it neglects so many factors such as race, bone density, muscle mass, etc. as compared to your favourite female idols? oh fuck that too - most lead extremely unhealthy lifestyles with extreme diets coupled with strenuous physical activity (relentless workouts like the lsfm trainee workout, plus dance practice all day). clothes sizes? different sizes fit differently on different body types, and different brand have different standards for their sizing - especially when it comes to clothes form the "women's" section. an m from one brand may fit as well as an xs from another.
people may be telling you you're overweight with the very best of intentions. they may genuinely want you to put effort into leading a healthy lifestyle. but they're putting undue focus on your weight, when frankly that's one of the last things one should focus on when trying to lead a healthy lifestyle. if you feel that your habits are unhealthy, focus on being healthier. don't focus on some numbers on some scale.
@Devil gave some wonderful advice here, and i'll only add on by advising you to put in some form of physical exercise regularly, and drinking tons of water.
i know i may sound very preachy right not, but trust me, this is all coming from someone who has struggled with accepting her weight almost her entire life. i'm not all that much older than you, and i still struggle with sticking to the healthy habits i mentioned. but really, it's important to try, but most of all, accept yourself. please take care of yourself, and again, don't take what others say to heart.