How to lose 70 pounds in 5 months?

hi my name is stacy, im on my moms yahoo account... im 11 years old, and i weigh 173 and im 5'1. i would like to lose 70 pounds before school starts next year, because im home-schooled because i am not confident about my weight right now... i know it is possible to do this, because my mom has lost 20 pounds in 1 week, so don't say take it 1 pound per month, because you can lose as much as you want. any suggestions on how i start? i LOVE salad, and i don't really like sweets anymore, and i excersize about 2 hours a day, stretch every morning for about 20 minutes, ride my bik a lot of times a day, and take a 40 minute walk every night before bed... i lost 5 pounds in one day, but then i gained it back, how do i lose the weight, and keep it. because i wanna be able to go to school next year and feel better about my weight [: