Do those kids that get in with the lower scores get enough tutoring help to be able to keep their grades up since they’re in classes geared towards the more academically gifted students or is it a constant struggle to remain eligible and obtain a degree?
A good question, but I think every student has to address his situation individually. You need to talk with coaches and schools about graduation rates, resources, expectations... Some schools provide tutoring and other academic support specifically for athletes. So far as I know, the schools I would consider true HAs do not--but most of those schools make quite a lot of tutoring and other kinds of support freely available to all students. (It's also true that there is no one definition of "High Academic School," so it's easy to end up comparing apples and oranges when discussing what "HA" institutions do.)
Similarly, the schools I'd call true HA generally don't limit what players may choose as a major. At a Johns Hopkins or Dartmouth you'll find plenty of pre-meds and engineers on any sports roster.
My bottom line is that many kids with 28 ACTs and 3.7 GPAs can succeed at top-tier colleges if they have the right mind set. These schools openly state that they every year they could fill their freshman classes many times over with highly qualified students. Even for kids with perfect grades and test scores, to some extent getting in is a roll of the dice among others with qualifications that are just as good. Many factors are beyond an applicant's control: Are they looking for students from your state? Have they already admitted 4 others from your high school during ED and want to get admits from other institutions? Particularly at the top schools, admissions is a black box: You need certain qualifications to get into the pool, but from there you really can't predict who will be selected. I think it's a myth to assume that the kids who enroll are necessarily going to perform better in the college classroom than those who aren't accepted (and admissions staff generally don't make that claim).
So if your son is admitted with a 27 or 28 ACT and ends up sitting in a room with 40 other kids who earned 34 or higher, can he compete? IMO it depends on how motivated your kid is, also on whether he just isn't great at standardized tests, how good his high school preparation was, etc. I went to an Ivy and I firmly believe that getting admitted is the hardest part. Once you are in, if you put in the work you will not fail out. And grades at top schools tend to be As and Bs--not many profs award Cs, Ds or Fs. (There are schools that are exceptions to this rule... Again, individual circumstances vary.) At Harvard, for example, the average GPA of the 2016 graduating class was 3.65 and the median was 3.70 (yes, on a 4-point scale). These schools are not using a grading distribution where the middle 40% get Cs and 10% fail out.
That leaves the question of whether a person is better off graduating with a 3.3 average from a HA or 3.8 from a "lesser" school. Again, I'd say it depends on the individual and his circumstances.
Sorry for the long answer to say "it depends." But I do think there is no one-size-fits-all way to approach this.