Typically when applying to a school, whether it’s high school or college, you will assume that the people reading your application will be primarily focused on your grades. This is definitely a major concern, but they are actually looking for well-rounded students. Employers/schools want to see what you do in your spare time to improve yourself, your community, etc. Schools and colleges emphasize the importance of extracurriculars. A lot can be learned outside of the classroom. If you are joining a sports team, you will learn the value of teamwork and physical strength/endurance. If you are joining a club with a mission (like competitive math), you will learn the value of individual and group responsibility. If you are joining a social/global or cultural club, you will learn the values of diversity and get a sense of community and culture. Most of these things are not lessons you can learn solely from your textbook. Don’t overlook the importance of extracurriculars.
Think of it this way; if you do nothing but attend school, you won’t have much to talk about in an interview. You have to make sure that you are studying, but also growing into the person you’re meant to be. However, this is a double-edged sword because if your grades drop because of it, that will be the first thing they notice.