Okiale – I’m not sure what you mean by a college prep school. Both Chofetz Chaim and Ner Yisrael have secular subjects and grant a high school diploma. I am sure that the secular curriculum they offer is enough to get into college since I think that many (maybe most?) of their graduates attend college.
Also as Writer Soul mentioned, most people who want to are able to get into top-notch colleges with a standard Yeshiva education that includes secular subjects. I have a sister who got into Barnard and Harvard Law School with a Bais Yaakov education. I had a classmate who went to Harvard with a Bais Yaakov education, and I know a boy who went to Harvard Law School from Chofetz Chaim. Not that I’m recommending that you go to Harvard; I’m just pointing out that I don’t think that going to Chofetz Chaim or Ner Yisrael would stop you from being able to. You don’t need an incredible secular education to get into college. If you go to a City College, you don’t need much. And even if you want to get into an Ivy League college, all you need are motivation and good SAT scores.