Everyone is born with, for want of a better term, a vessel labeled "potential." The vessels are all different sizes, but all begin empty, and begin to fill as life moves forward. The object is to get your child to fill the vessel (potential) he/she was given. (You can't fill it more then it is able to contain.)
Set the bar too low and a kid coasts and doesn't need to find out the limits of the potential; set it too high and you may have a houseful of unnecessary stress. Only you can know your child, his abilities, his potential, his weaknesses, his "issues" (and each one has his "issues").
We set the bar very very high, knowing we could always lower the bar if we believed it necessary. (We lowered the bar in college once it became clear that our expectations were not doable.) The mantra we always used was to be able to answer this question: did you give an honest effort, did you lift up all the rocks to look underneath, did you leave it all on the field/classroom? If not, why not? (Sometimes there were reasonable explanations as to why max effort wasn't expended (e.g., when a grade was solidified to the point that it couldn't be moved regardless of a test score).) For us, the approach worked; but for others who would try the exact same approach it may not.
And the element which I believe has the greatest effect: parents who show they care about the learning, care about understanding the school's social scene, care about the teachers, care about the grades, and speak with the kids. There are no surprises in this scenario. Our school also had a great tool to keep on top of the academics: I forgot the name but it was an Internet based program which allowed the parents to be kept abreast of homework assignments, tests, grades, reading, etc.
As for equating GPAs between HSs, every school submits its HS profile to schools to which its students apply (ask the GC for a copy). The profile will show what classes are offered, average grades and average standardized scores. Thus, a kid applying in the regular pool from a school which offers few AP classes is not penalized simply because the neighboring HS offers dozens. What is important is the composition of a kid's HS schedule. So, for example a kid who took all three of the APs his HS offered will be viewed in a more favorable light when compared to a kid who took five out of the twenty offered at the neighboring HS. (That's what schools refer to as rigor.)
For the top schools, rigor is critical. Perfect grades without rigor gets a kid nowhere in those schools; conversely, mediocre grades in a rigorous schedule get a kid nowhere. As trite as is sounds, for unhooked kids (non-athletes, specific types of students, etc.) virtually perfect grades with the highest rigor is required. But keep in mind, even those kids have a chance in the single digits of getting accepted into those schools.
Test scores are another critical component. But even here, unless a kid has a 2400 or a 36, the scores act more a a disqualifier than a huge help. Yes, a perfect score is a huge deal (so long as it's coupled with perfect grades, and a healthy helping of the best ECs available to the kid).
For unhooked kids those top schools are a crapshoot no matter what. (As a example, I personally know a girl with the highest rigor possible, 3.98 unweighted, [she got one B in HS], took first place in the grand award section of the Intel Science and Engineering Fair (on a project developed solely by her and done solely in the HS lab - resulting in a patentable invention), multiple special awards at the same fair, 34 ACT, incredible high end ECs outside of the HS, editor-in-chief of both the year book and the literary magazine (both being recognized with awards by the city), etc., who was rejected (following a deferral) by Stanford.)
Circling to the original issue: set the bar high, very very high. Accept nothing short of your kid's best efforts academically or otherwise (video games excepted). fill that potential - whatever it is. This keeps options as open as possible.
Note: my comments are directed to kids not being recruited. For that lucky group, every school has lower requirements.