Jumping in from the "squid" perspective (and plebes don't get "grades" during Plebe Summer), one of THE hardest things for mids or cadets is first semester academic grades. USNA -- and I assume USMA -- gives Cs and Ds with frequency -- and even Fs. And these are students who are studying like crazy, giving it their all, getting EI, etc. At USNA, you can have a 3.25 and be in the top 15% of your class. Think about that for a minute. 85% of the class has less than a B+ average.
For kids who've always earned As and maybe an occasional B, this is a rude awakening. It was for me when my first semester came in as a 2.63. Yep, I still remember it.
I was shocked. My parents couldn't believe it, especially b/c they knew I was working harder than I'd ever worked in my life, academically speaking.
To an outsider such as myself, it seems that the Beast grades are good prep for Ac year and probably do cadets/NCs a favor by showing them early on that not everyone gets As. Or Bs.
For parents who don't have a SA background, realize that your cadet is very unlikely to replicate his/her h.s. academic success at USMA no matter how hard he/she works. Some will, but most won't.
Finally, let me say that it is REALLY hard for a mid/cadet to show those first SA grades (Beast or Ac year) to parents who are used to seeing all As. I was so ashamed of my C+ average in my first set of grades, even though I knew I'd worked like a demon just to get that. I know my parents were disappointed, even though they never said so. Thankfully, my grades improved over time -- but a lot of my classmates worked just as hard or harder, and were just as smart or smarter, and yet still struggled.