I was looking at the way my son's school calculates GPA and weighted GPA. Then I looked at how other schools calculate them. Since I was looking at high school web sites, I was only partially successful. In any case, many of them seem to be math-challenged. Even when I searched the web about how colleges calculate GPA (or not), I met with little success. We are told that colleges generally like to recalculate their own GPA. Why? If they have the numeric percentage grades, why do they have to sort them into GPA buckets (e.g. 83-87 is a 'B' = 3.0) and then come up with a new number that contains less information than the percentage grade. Perhaps some high schools send out only GPA bucket scores for each class, but do colleges know what the buckets are?
What I find more interesting are discussions that talk about how colleges like to use unweighted GPA scores. There is some sort of advantage to an 'A' in a regular course over a 'B' in an honors course. Maybe. I think it depends on whether you make it past an initial cut. Someone told me once that colleges use unweighted GPAs for the initial cut. That seems backwards to me. You want to use a weighted GPA (and rank and SAT and etc.) to make the initial cut, but then compare applicants based on unweighted GPA. Of course, they must first strip off all of the non-academic courses.
There is the Ivy League "Academic Index", which uses something called the "Converted Rank Score" (CRS) along with your SAT scores to come up with a number, that presumably, is used to determine whether you make it past a cutoff. The CRS number is based on your class rank and the size of the graduating class. Since most high schools rank based on a weighted GPA (including all fluff courses), this would indicate that class rank matters more than the unweighted GPA of core academic courses. That seems to be a poor game to play. If you are on the bubble, perhaps you might want to look at another college. I told my son that it might be nice to look at his GPA, but he should just try to get the best grades on his core academic courses. This came up because some kids at his high school are getting weird about GPA scores.
I do, however, tell him not to give away free points. For example, in science, they had a grade for whether they covered their books or not. I showed my son that if he didn't do it, his overall grade for the class would drop 3 points. I told him that it was like finding free money. Then there are the things you have no control over, like group grades. There was also the essay where he got an 80, but that was the second highest grade. Both of those things dropped his quarter grade by 5 points. There is not much you can do if a school has two different grading rules for the same course and you stuck with the wrong teacher. Things might average out in the end, but it's difficult while the student is going through it.