It’s school first

With all the talk about what colleges look for in applicants, it helps to remember that college is school first. 

Yes, colleges care about your extra-curricular activities.  They care about how you're going to make contributions to the campus.  They like it when your interviewer finds you personable, and when you show interest in the campus, and when you tell engaging stories in your college essays. 

But none of that is as important as how you demonstrate your likelihood of being an engaged and successful student in college.  And you do that with a rigorous high school curriculum, good grades, and a demonstrated interest in learning. 

An admissions officer is far less likely to be impressed by reports of your pitching arm if your academic work is substandard.  They'll find another applicant who can throw a good fastball but also has rigorous classes and good grades.

"My grades dropped because I was so committed to my extracurricular activities" is not a phrase that's going to move most admissions officers. 

A good audition, a letter from an influential alumni, a hundred hours of community service–those things are fine, but they won't make up for academic shortcomings.  

I'm not suggesting that you bury your head in the books and refuse to come up for air for four years.  Enjoy your activities.  See your friends.  Have fun.  Get enough sleep.  Commit yourself to things you enjoy.  Occasionally do things that have nothing to do with college admissions or improving yourself.  Goof off every now and then.  Be a nice person to teachers and to other students.  Fill your high school years with emotional growth and good memories.

But while you're doing those things, challenge yourself academically and work hard to meet those challenges.  And feed your intellect by learning more about the subjects that interest you most.

Your academics are the first place an admissions officer will look to assess your academic readiness for what college is first–school.