At Northwestern Prep, the weaknesses of your application will become very apparent, as you are starting the application process over from scratch while making significant improvements. To make sure you understand where your application was weak/needed improvement, the director of admissions herself makes an effort to visit the school and sit down with each student 1 on 1 to let them know where they need to improve.
As far as options for prep schools go, there are quite a few prep schools around the country that have successful programs, you just have to research. The ones that seem to have the highest success rates are NMMI, MMI in Alabama, Greystone Prep in Texas, and NWP in California, the latter being the best in my (biased) opinion.