In my experience, the key to losing weight is to love the activity you're doing more than you want to lose weight. It won't erase your insecurities overnight, but it will make them more manageable than if you obsess about your weight and allow negative perceptions to influence your eating/exercise/sleep habits (and all of that matters!).
Eat well, lift, run as much as you can, and do it with people you love and who push you. Start with the coach. That behavior is not acceptable. Once that's addressed, I recommend sitting down individually with everyone on the team who has made derogatory comments about your weight or a bystander to them and being direct, polite, but firm in your explanation of how their behavior affects your well being, not only as their teammate but as a person. If they sincerely apologize and change their behavior then they are worth giving a chance. If not, f*ck them, they aren't worth trying to convince. Either way, confronting the team individually and in person will earn you respect and as you progress throughout the season. Definitely do not do the same with the coach, as other posters mentioned. You need to speak to both your parents and the Athletics Director at your school about that behavior and let them handle that.
I've had it, from classmates, a handful of teammates, superiors, teachers/instructors, the whole nine yards. My hope for you is that regardless of how this situation plays out you realize that your self worth is not tied to a number on a scale, but your general health (which is not exclusive to your weight) and how you deal with situations like this (I understand the helplessness but LetsRun is not the place for this, especially if you're underage) is far more a reflection of your worth than any PR, slight decrease/increase in weight, or inflammatory comment. It gets better, but that comes from your efforts to improve yourself and handle/regulate your health (mental and physical), and regardless of how things go with your teammates and coach, you will find no peace until you make yourself happy first.
Best of luck. Stay tough.