TAKS doesn't stop your grade advancement, but it does stop your graduation if you cannot pass the exit level. Texas (and pretty much every state, thanks to NCLB) ignores any safeguards or necessary steps to fix the problem for kids who fail in 3rd, 5th, 8th, and the other "main" testing grade bumps.
No diploma = no entry to schools in other states, either.
But, like I said, it's nearly "impossible" to fail out the max times with the amount of help some of these kids get. Some athletes are classified as 504 or "accommodated" so they can have all the questions read to them, in a voice that might "gently lead" them to the proper response. I know with 100 percent certainty that I've met a UT athlete who fit that description, and if there's one, there's more.
As for the students who aren't strong or fast, well, they're SOL.