First, the disclaimer: I am an instructor at Marion, and been here 17 years. I'll try not to put on the sales job, though, because your choice in college should fit you and your needs.
LG is right in that you do not have to go to a prep school when you reapply at an SA. If you have a strong collegiate record (A's in SA freshman year classes), then you will have shown success and helped out your application. He's also right in that you likely should improve your SAT/ACT scores. Taking the tests again (and again!) can help, though a prep course, either online or paper, would be a benefit.
If you go to a prep school, though (not just MMI), you will have instructors who are familiar both with the SA courses and the SA appointment process. Small class sizes, accessible instructors, and liaison with the various academies are also selling points for prep schools.
At a military prep school, you also get the opportunity to participate in a corps of cadets. This is your chance to try on the SA experience and see if it fits you. There are also leadership opportunities available within a corps that are not available in a university or community college. For example, in the past three years I've had two company commanders, one company exec, two platoon leaders, and a company first sergeant selected out of the SAP students I advise. Many do become squad leaders and team leaders, so your leadership potential can be quickly identified and demonstrated.
The end result is up to you, though. Regardless of your path, you have to work hard, manage your time properly, be aggressive in seeking out help, and look for opportunities to demonstrate why you should be selected for an SA.
Best of luck to you, and good luck in your application process!