Getting merit is difficult (if it exists) and you are competing with seniors who have more experiences and higher scores/grades. I also think that schools will not want to give merit to a student who will only be 16 when they graduate.
What is the rush? Medical school (among other paths) requires maturity and experience. While I am not suggesting that you are not mature for your age….I am suggesting that it is not the same as waiting and graduating with your peers.
My daughter chose a career path where interviews (if you received one) for acceptance to a program were intense. She had full-day interviews, multiple full-day interviews over 2 consecutive days, and full-day group interviews and situations. The questions and problems she was expected to discuss and navigate would have been impossible if she did not have the experiences that she did.
I cannot imagine attempting this at the age of 19 or 20. High scores and grades are not enough- there is a lot more to consider.
I would not rush the process.