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<p>Those students are not impressive compared to [this</a> student](<a href=“15-Year-Old Boy Set to Graduate from UCLA – NBC Los Angeles”>15-Year-Old Boy Set to Graduate from UCLA – NBC Los Angeles) who started at community college at age 8, transferred to a state university at age 12, and graduated at age 15.</p>
<p>Seriously, super-elite students at elite high schools are outliers; taking multivariable calculus (which is beyond the AP / college-frosh level) while in high school is beyond what most high school students, including most aiming for highly selective universities, reach.</p>
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<p>Most universities will not consider an applicant a transfer applicant unless s/he takes college courses after graduating from high school (exact thresholds vary; check web sites or ask directly).</p>
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<p>While many private schools find ways to exclude transfer credit (presumably, they want you to stay and pay tuition as long as possible), you do want to check that you can get appropriate subject credit and advanced placement for your prior course work, so that you do not have to repeat (for example) calculus 1, 2, 3 etc. before taking more advanced math courses.</p>
<p>Public schools tend to be more generous with transfer credit as far as credit units go (since most of their students are subsidized for lower in-state tuition, they want the students to graduate quickly), but subject credit and advanced placement need to be checked using the articulation agreements (usually between community colleges and same state public universities), or evaluated individually.</p>