Several weeks ago I was watching a MIT lecture, and the professor said something along the lines of this:
“I used to be the admission chair thingy, and we would look through these transcripts, saying ‘yes’, ‘no’, ‘hell no’ puts down imaginary paper, you guys have the brains to succeed in this course etc.”
what sort of transcript would be considered good for an MIT applicant? Is one B acceptable? 2 or 3 in the first semester (but with an upwards trend?) One C in a foreign language?
What would a borderline “no” look like, and is explanation of certain grades possible in an application supplement given that SATs scores are pretty high?
Thanks! 
Edit: I take IGCSE and IB, so A, Bs and Cs, might mean different things to A Bs and Cs in the US
As a mom with no direct MIT experience, but who has done a TON of research on MIT’s admissions process, I can safely say that they are not looking for a hard and fast grade letter formula. What MIT cares about is that you’ve taken the most rigorous courses you can (and IB is that), and that you did as well as possible. If your grades dip, there’s a place on the app where you can explain any issues.
MIT truly cares about the whole student, so grades and transcripts are one piece only of the puzzle. They are interested in passionate, curious thinkers, and not grade-grubbing automatons with arrogant attitudes (not saying you’re one!). While other schools may say this, MIT truly embodies it. This is a large reason why they’ve become so popular in the last decade or so, and admits are around 5-7%.
I urge you to check out the MIT Admissions blogs-they are amazing in their humor, honesty, and transparency.
Good luck!