Ms. Passionate About Education

Though your GMAT score is weak, especially for an international applicant, other aspects of your profile are as good-as-they-come: highest-tier employer (McKinsey) with standard number years work experience with sponsored status (i.e. 100% job security at a top job and nearly 100% certain tuition payment), STEM undergrad, relatively underrepresented international applicant (Brazilian female), and ambitious professional aspirations that also happen to have a profoundly positive social impact.
Have you attempted the GRE instead? There are people who favor one test over the other so it’s worth finding out if you might perform better on a relative basis on the GRE. HBS accepts either of them. There are lots of success stories of people who beat the odds with a sub-700 GMAT score and obtain admission into HBS, but it’s very difficult. People with a sub-700 GMAT score generally have some other signal that they can keep up and thrive in a rigorous academic environment (e.g., great grades in a challenging STEM-y academic past such as electrical engineering or physics or math, prior excellence in medical school, CFA designation, etc.). I’m thinking of one of the HBS alums with your GMAT score who was a medical school dropout (grades weren’t the problem… she performed excellently academically, she just decided against becoming a physician) AND excellent undergraduate credentials in STEM that got her into medical school in the first place. She didn’t do well on the GMAT for whatever reason, but the other academic accolades carried her so that it was obvious that she could thrive in an HBS classroom. (Obviously, like any other HBS student, she also had amazing professional accomplishments after dropping out of medical school). Your GPA of 3.0 in engineering will be viewed as respectable but not amazing and below average to a US AdCom who is accustomed to grade inflation and a 4.0 scale. Sometimes international applicants, especially those from LatAm or India (geographies where grades are less inflated, especially for students at top-tier universities), provide context that can bolster a GPA that is optically lower. For instance, if a 3.0 actually put you in the top 20% of your class or on a “Dean’s List” or some other relatively high ranking compared to your peers, it’s worth mentioning that.
You’ll doubtlessly have great essays re applicable work experience from your time at McK, so we’ll want to flex what differentiates you outside of work. The endowment that you started sounds ripe to explore, though I’m worried that UG experience may appear dated to the AdCom for an applicant that graduated 3-4 years ago. International applicants are often unfamiliar with the extracurricular arms race of U.S. graduate school admissions. Thriving professional clubs, an active social scene, and plugged-in alums who want to help current students are some of the things that make MBA communities vibrant places to spend two years. AdComs want to admit candidates who will engage with their community outside of the classroom: students who will lead the marketing analytics club, who will coordinate the first-ever edtech conference, who will use their personal networks to bring speakers on campus for lunch-and-learns, who will graduate and be active forces in philanthropy and civic efforts in their communities. They want students and alums who will “lift as they climb.” The best way to signal that you will be this type of future MBA student and alum is to SHOW RATHER THAN TELL that you are already this person today. I.e., that you are engaged in activities and passions outside of work. If you can demonstrate this well, it will signal that you will be engaged in leadership outside of the classroom at school and can juggle lots of competing interests and responsibilities.