It's a common dilemma for high school students: should I spend my summer studying for the SATs or doing an internship? The answer, as is often the case, is a balance.
The Foundation: Grades
Let's be clear: grades matter. They are the primary indicator of your ability to handle college-level coursework. If your grades are slipping, an internship won't save your application.
The Differentiator: Internships
However, once you cross the academic threshold, grades yield diminishing returns. A 4.0 GPA isn't significantly better than a 3.9 in the eyes of many admissions committees if the 3.9 student has built a mobile app, started a non-profit, or interned at a venture capital firm.
Synergy
The best approach is to find synergy between your academics and your extracurriculars. If you excel in math, a finance internship allows you to apply those skills. If you love writing, a content marketing role at a startup gives your passion a platform.
An internship shouldn't come at the expense of your grades, but it should be a key component of your holistic profile.