Teacher recommendations and the essays that you submit to the colleges are also important in admissions, and the main channel through which human capital not particularly captured by grades, and personal development are signaled.

Teacher recommendations and essays may be weak signals.

Getting good recommendations depends in part on how appealing you are to teachers (in respects that are orthogonal to personal development). For example, people develop halos around people who are physically attractive, viewing them in more favorable terms along all dimensions.

Some students get extensive coaching on their essays.

I basically completely ignored signalling while in high school, and often prioritized taking more interesting non-AP classes over AP classes, and focused on a couple of extracirricular relationships rather than diversifying and taking many. My grades and standardized test scores also suffered as a result of my investment in my robotics team.

I'm interested by this in juxtaposition with the fact that you got into Harvard. If you'd be willing to email me with some more details about your personal profile (e.g. high school grades, test scores and extracurricular achievements) I'd very much appreciate it. You can reach me at jsinick@gmail.com.