Further SDI teaches instructors that the ultimate standard is "would you feel comfortable having this person dive with a member of your family?" I personally don't have any children, so I always look at my students and ask myself, "would I be okay with sending this person to go dive with one of my nieces?"
I have had students that didn't want to be there. Most recently, I had a teenage girl in the classroom. She was getting certified along with her mom, because her mom was making her do it. It was evident that she had no interest and would not put any effort in. I told the mom that if the daughter didn't want to do it, and didn't demonstrate the knowledge and skills, she was not going to get certified. At my shop, there is no pressure to certify anyone that I or any other instructor is not comfortable with. And, we never have more than 4 students per instructor in open water and usually not in the pool, either.
In other words, the instructor is the final authority (at least, with SDI/TDI), and so, if an instructor certifies a bad diver, that is on the instructor. If the student doesn't want to be there, doesn't put in the effort, and is not a safe, competent diver (at the new, Open Water level), then the instructor should not issue a certification.
Of course, I recognize that it does happen that sometimes we'll have a student that does great, gets certified, then goes out into the real world, maybe doesn't dive at all for a while, forgets everything, and then they are a terrible diver. But, I am not sure there is any standard or agency that can really fix that issue.