We don't have SAT in Canada and mostly rely on high school grades. Most universities/colleges adjust student GPAs on a per high school basis based on alumni performance in their first year courses.
I personally prefer the GPA system as it measures your performance over your final 2 years of high school rather than having your future determined by one test.
You can retake the test as many times as you want. Some schools average them, others will take the highest. The whole point of creating the test(s) in the first place were to deal with variability of GPA's between schools, so it's a bit useless to say "just adjust GPA's between schools instead".
In the US now we have federal school testing to try and normalize schools better. But before that many schools were graduate students that were illiterate (actually some still are, but I hope it's gotten better..). The US is 10x the population of Canada so image the difficulty for a liberal arts school in the northeast looking at a student from some rural 500-person school they've never heard of from the southwest.
Adjusting GPA between schools is one approach to deal with grade inflation between schools. SAT is just another approach with its own tradeoffs. I've simply listed why I prefer the former over the latter approach.
Canada's population is smaller but we also have many obscure rural schools. Non-prestigious universities don't have much data but they also don't have much choice in top applicants so it doesn't really matter. Over time, they can build up decently accurate profiles of high schools for majority of applicants.
They do a regression that includes both GPA and SAT. That is mathematically an adjustment of grades, where the adjustment is the SAT term. Then, as articles such as this one show, this correction correlates well with success at the university. So ultimately it becomes the same correction you want.
It would be interesting to see a study that compares SAT scores vs alumni performance to see which one has a better correlation with success to use for said adjustment
It's been many decades since I took the SAT (3x, at ages 11, 12, and 16), but I seem to recall that I had a choice of which schools to share my SAT results with, such that a school that only ever saw my age 16 SAT test couldn't possibly average in the other two scores.
It creates bad outcomes where students retake classes in the summers as a profession and game the system.
It also puts you at a disadvantage if you come from an immigrant area or poor area where your 90% will not count as highly as someone with 60% from the richest areas with a trackwork of success.
It really punishes anyone not rich and locks them out of elite universities and pushes most of the poor out of average universities.
A single test where everyone can study and pass seems more fair than basing it on things that can be impossible to change like where you grew up
Said test can also bias against the poor depending on how it's created. It can ask questions based on subtleties that only private schools/tutors have the time to cover or on life experiences that only upper middle class realistically have experienced.
Neither system is perfect but I think the purely SAT approach creates too much unnecessary stress for high school students when there are other proxies for determining future success.
There is nothing stopping you from taking the SATs multiple times. It's encouraged because you most likely will go up a few points the 2nd pass. I've taken it twice myself, 3 times if you include the PSAT. And I also took the ACT, only once.
GPAs you can only screw up once. If you fail even one class it's impossible to climb back to the top.
I personally prefer the GPA system as it measures your performance over your final 2 years of high school rather than having your future determined by one test.