You should assume that the overwhelming majority of applicants to Brown and Stanford are in the top 10% of their class. “Top student in the high school” will not be all that unusual for applicants to Brown and Stanford (and some other top schools on the Harvard/MIT/… sort of level).
You do not need to attend a highly ranked university to have a good chance to get admitted to a very good graduate program or a very good medical school. Also, if you are going to be taking premed classes, these classes will be very academically demanding and full of very strong students at a wide range of universities and liberal arts colleges. A’s are not easy to get in premed classes, but quite a few A’s will be needed to keep open the opportunity to get accepted to medical school (a few B’s are okay also, but still will not be easy to get in some cases).
One daughter started off as an environmental science major at a university that is barely NOT ranked in the top 100 in the US. One good thing about starting as an environmental science major is that she got to take classes in multiple different sciences, which helped her pick one science that she then switched to. This at least from slightly afar looked to be quite a graceful and seamless transition. She also was completing the premed required classes at the same time, but with the intention of possibly being a veterinarian rather than a human doctor. The required pre-vet undergraduate classes are the same as the required premed undergraduate classes and her pre-vet classes were full of premed students (some electives such as “lameness in horses” were different). Her “medical experience” on the side was all veterinary experience. Graduating from a “not quite top 100” undergraduate university she did very well on DVM admissions and is now studying at a very good (and well ranked) DVM program and is doing well.
If medical school is a possibility at all, you should pay attention to budget and do your best to avoid debt for your bachelor’s. It would be even better to save some money in a college fund for medical school (or for a master’s). Medical school is likely to cost $100,000 per year (or more?) by the time that you get there. If your budget is anything less than full pay you should run the NPCs (you will need help from your parents to do this).
One thought: McGill is noted for grade deflation. If you are seriously interested in keeping open the option of medical school, I would be concerned about this so I might question or even take back my suggestion from above.
I think that it is worth applying to Brown and Stanford, assuming that the NPC results are okay. However, you also should be thinking carefully about affordable safeties to apply to, where you would be comfortable studying for 4 years.