Joining the Marine Corp -- MEPS?

I'm attempting to join the Marine Corp right now and I'm worried I may not p the IST and/or weight requirement at MEPS. I'm sure I can p everything else as far as the medical exams and physical, but surprisingly the IST may be the show stopper for me. I'm only a junior in high school and I need to wait until I get my diploma next year before I can leave, so my recruiter wants to sign me up on the DEP. If I were to go ahead and signup for DEP, how long would I have to get in shape before I would go to MEPS to get processed to see if I qualify?