This link will get you to a nutrition expert, Renee Radenberger. It may take her a few days to answer your questions, but she's an expert and I'm sure she can give you some guidance on 1) identifying how much you should weigh and 2) advising you on the best ways to reach that weight. Good luck!
http://www.medhelp.org/forums/Child-Nutrition/show/142
20 pounds is too much to lose in a month---it can be done, but a big portion of the loss will be water and not fat and that kind of loss is often only temporary. (Take it from someone who's lose 75-100 lbs. 4-5 times in her life.) If you cut your calories TOO low, your body will think it is starving and cling to every calorie you eat making it even harder to lose weight. You should ask your parents to bring you to your pediatrician and have your doctor give you advice about healthy eating and weight reduction. Or, even though I know you want to lose the weight before school, see if your school nurse can refer you to the school nutritionist. Eat lots of veggies and lean meats...avoid candy and foods that have high fructose syrups; avoid "processed"/packaged foods & eat "fresh" foods when you can. If given a choice between an orange or apple and orange or apple juice, take the fruit to get the benefit of nutrients that are removed in the juice making process. Don't spend too much time in front of the TV or computer: get outside & be active. Find sports that you enjoy sharing with friends or activities you enjoy alone. Search this site for "teen AND nutrition"...there are some EXPERT doctors & nutritionists who are better qualified than I am to advise you. And don't focus TOO much on your weight. It's important to achieve and maintain a healthy weight, but you are worth SO MUCH MORE than the number on the scale. Don't let a number on a scale make you feel bad about yourself, EVER!
anorexic people eat too little, belemics binge and purge, and dropping your calorie intake too low is a SIGN of anorexia, but doesn't mean u r. Eat the calories your comfortable with, but cut your portions of red meat, carbohydrates, and starches (like potatoes) in half at least! fill up on fruit, veggies, beans, drink plenty of water and cranberry juice, but not too much. everything in moderation is key! even carbs. for faster weight loss you want exercise, but if you're not used too a lot of activity, keep it simple and start slow because you don't want to have a heart attach b4 school starts, right? jog through sand, slowly to prolong your endurance and lengthen the exercise period, you want to get you heart pumping, but at a pace only a little faster than normal and level it off where you can maintain the same pace for about 10-15 mins. walk up hill again, slowly and with control, feel the muscles working and focus on each step. google 'yoga for weight loss' for more ideas, it's low impact so your not gonna drop dead, but the results are awesome! it works fast and you feel great, not tired and you can continue being more active all day, not just for the half hour you work out
good luck