Should I quit my sports team to focus on the SAT?

I am on my school basketball team, and there is a 50% chance that I'll be co-captain next year. Unfortunately, my grades aren’t that great, so I really need to study and do well on the SAT.

Should I stay on the basketball team, possibly as co-captain, even though it’s so time consuming, or quit and make sure I ace the SAT? What will look better to colleges?

In general, your SAT scores will matter more to colleges than your sports participation unless you're an absolutely outstanding athlete. That being said, you shouldn't sacrifice an activity that you enjoy just to improve your performance on a standardized test. Basketball season typically only lasts for a couple of months in the winter. If possible, plan to take the SAT either before the start of the season or a while after it ends so that you'll have more time to study.

This year, you might decide to take the SAT on October 1st or November 5th. That still gives you the whole summer to study before the test. If you wanted to wait and take it after basketball season, you could plan for the May 6th or June 3rd test dates in 2017. If you chose one of those dates, you could finish up the season and then still have a few months to study. You won't need to put in ridiculous study hours to prepare for the SAT if you plan wisely.

One qualifier: I answered this question under the assumption that you really enjoy basketball. If all it does is stress you out and make it more difficult to accomplish your goals, then quitting is always an option. Don't feel obligated to continue an extracurricular activity that has a significant negative impact on your quality of life.