The bolded is hard to believe for me. You can easily have students who take easier courses to get higher GPAs or teachers who are lenient with their scoring resulting in higher GPAs. That doesn't mean those students are "college ready" either.
Again, standardized testing has been around for decades. There's nothing new there. It's only the pressure/stress that is put on these kids (whether by schools, parents, or themselves) surrounding these tests that's "new". Why not fight to eliminate the pressure instead of fighting to eliminate the tests?
This is the most recent and largest study
http://www.nacacnet.org/research/research-data/nacac-research/Documents/DefiningPromise.pdf
And an NPR recap of it http://www.npr.org/2014/02/18/277059528/college-applicants-sweat-the-sats-perhaps-they-shouldn-t
Your first paragraph is very interesting however, you are assuming the worst about every student and every teacher. Why would you jump to that conclusion? Why do you assume a majority of teachers would be more lenient? That seems to me to be a very pessimistic view of students and teachers. I much prefer the alternative where most teachers are fair and balanced and most students succeed because of their own hard work.