How can you get into an Ivy League school, MIT or Stanford with some of your high school grades being mediocre?

With those grades, you need some outstanding extracurriculars (state champ at a sport, international science fair competitor, patents, etc.) and a fantastic application that sets you apart from the thousands of applicants who have 4.0 GPAs and 2300+ SAT scores. You can also try to play up how you would add diversity or unique experiences to the student body of the school. If you do have such experiences, Stanford does tend to value extracurriculars more than other top schools, so you can give that a shot. Otherwise, you at best have a shot at the "lower" Ivy's, and even then, you can't realistically get your hopes up. You are competing with literally thousands of applicants with close to perfect GPA and SAT scores (many of whom are going to get rejected), and from what it sounds like, you are not going to be too competitive with them. From personal experience, my 3.95 (4.0 scale) /2250 got me rejected from Harvard, Yale, Duke, Johns Hopkins and probably MIT and Stanford as well if I had applied. And this was 2006, its only gotten worse since then. The top 2 schools I got into to were Cornell and Wash U St Louis

As implies, the front door to schools like Stanford is probably locked. So figure out how to get in via the side door (or other unorthodox routes, as in the colorful phrase http://www.merriam-webster.com/dictionary/second-story%20man) Your best shot is probably as a junior-year transfer. Schools like Stanford take a few extra people partway through the college process, restocking each year's class to make up for minor attrition of the original entering class due to poor grades, family upheavals, etc.) Think of this as a chance to make a fresh start at a slightly less famous school that might springboard you into the elite school of your dreams. Take hard classes. Get As, again and again. Do something eye-catching in your field -- original research if you're in the sciences; published poems, essays, etc. if you're in the humanities; heroic civic work if you're in the social sciences. Now you won't be asking the elite schools to believe that your slow start masks your eventual rise to greatness. You will be showing your emerging greatness in many ways. There's only one catch. If you do all that, you may decide that you don't need Stanford. There's a legendary research study by Spencer Stuart (the recruiting firm) and Carol Hymowitz of The Wall Street Journal, looking various CEOs' academic origins. Guess what? The best proving ground for future CEOs turned out to be the University of Wisconsin. Granted, it has a larger student body than Stanford or Harvard, but still .... Here's the study:  http://online.wsj.com/article/SB115853818747665842.html

There are tons of students with straight A's and near perfect SATs who don't get into the top schools.  For perspective I was valedictorian of my class of 800 students, had perfect SAT's and no grades below an A; in terms of extracurriculars, I was a state champion debater.  I got rejected from Harvard, Yale, and Princeton. I am currently a Stanford student and have never heard that Stanford cares more about ECs.  If any bias, they seem to accept more students who do well in math and science, than languages or history.  I doubt anyone with a transcript with two D's would make it.  Kick ass in a lower ranked college, save yourself the cash, and ultimately it won't matter as long as you develop useful skills.  Tons of Stanford students end up being useless and sometimes broke.  Focus on what you want to do career wise - not what college you go to. Also - just because you might not get into a top college doesn't mean you shouldnt continue to work hard in high school.  Knowledge is CUMULATIVE and the more you push yourself now the faster you'll grow! As an aside there are particular schools that are AMAZING for certain programs but not *top* all around.  University of Washington for example has a top notch CS program but is not as competitive as Stanford to get into.  But UW programmers do super well in Silicon Valley too.  NYU which is obviously super competitive and would probably be a reach but not *as* competitive as the Ivies, regularly feeds into top finance jobs.  Notre Dame also tends to do surprisingly well in finance placement.

You probably can't -- for your undergraduate degree.  The competition is just too tough.  But if you do really well wherever you do go for your BS degree, AND pick a demanding major like engineering or science, then you might get into a top grad school. However, you need to think long and hard about why your high school grades were mixed -- and how you can change that.  Any undergraduate major hard enough to get you into a top grad school is gonna be much more challenging than your high school classes were.  Engineering school is pretty challenging even for students who worked hard throughout high school; I know this first hand.