Wanna loose wieght fast but then maintain it and change lifestyle?

basically i have gained about 20 pounds since summer 2008. which is upsetting i exercise aloooottt.. actually at the beginign of summer in may i was 162 and doing well being 5'8 i can carry weight with out it lookin bad. but since may i have gained another 10 pounds and its startign to depress me cus its startin to catch up to my height and its noticable. i dont want to go back to school 10 lbs heavier... if i workout twice a day and change my diet is it possible to loose about 12 lbs by aug 31st in time for school?