Don't plan to get into MIT chemistry this far ahead. It is very competitive to get into grad school at MIT and equivalent places, not only do you need to have excelled in the classroom, you also should have done good research as an undergrad and be able to get great recommendation letters. You haven't even completed high school, how do you even have any idea if you will be a competitive grad school applicant at this point.
How do you even know you will still be interested in chemistry when you get to/finish undergrad? When I was 14 thought I wanted to get an MD/PhD and study molecular biology. Now I am doing my PhD in theoretical condensed matter physics and am interested in quantum phenomena in matter (became interested in quantum from high school chemistry actually and decided to study physics after my honors course freshman year in college). Keep an open mind, you never know what you will learn and experience between now and then that might change your mind (my interest in my subfield was first developed talking with one of my professors).
As far as your question about workload at MIT, I have a lot of friends who went to MIT for undergrad or are getting there PhDs there in physics or chemistry. I myself go to a similar institution. MIT is an incredibly intense place, there a tons of brilliant students who do amazing things there. Not only is the course load challenging, grad students spend a lot of time doing research.