Neither SAT nor ACT is measured in "grades".
This doesn't somewhat improve college performance. It creates a markedly different brain and habits. An effect that declines as entrance into such a school system becomes later. Middle school being exceptionally late. High school being so late so as to have a marginal effect if any. Kindergarten being optimal.
Prep schools, generally speaking, only give basic test instruction for college entrance exams. Other standardized testing is the same or even less than normal schools. Cram-course style instruction is generally private and paid for privately by any given family. In my experience, the vast majority of prep school students don't seek it. This is in the United States.
Not if they go to a school that doesn't have the prerequisites for college/university. Not if the school is so poorly-resourced that they don't have access to the study material, or even access to take the standardized tests they need. Not if the teachers are jerks. Go luck getting into a university science program if your highschool didn't teach science.
(Yes, that is a thing even in the US. Many religious schools do not teach what we who read HN would call "science", which makes university applications tricky.)
Universities in the UK use A-level results (which are mostly based on standardised tests), but will sometimes compensate (lower) the required grade based on a students school and background. Which IMO works out fairer in practice.