Most high schools in the US don't have graduation exams. Even for ones that do, there's no standardized high school graduation exam nationwide.
So how do people fail to graduate high-school?
If the bar sounds low and subjective, that’s because it often is, and as such I think it’s mostly useless for colleges looking to measure students against each other to choose who to admit.
Most public High Schools in the US are honestly extremely hard to fail out of. As long as you get C's in all your classes you'll graduate.
* Graded homework
* Papers/Project/Labs
* Midterm/Final Exam
The teacher chooses the exact mix per class, so it's possible you fail even if you ace the exams, or pass even if you fail the exams. This variability is why the standardized tests exist.
The exact weightings are not known to the students applying, but it's safe to say you can't just minmax on the standardized test.
I don't know if you've picked up the tone of some of the other comments in this thread, but people feel very strongly about what factors should be considered in college admissions. Almost no people will defend legacy admissions, few will defend athletic admissions, some will defend musicians, many will defend GPA and standardized testing.
College admissions are zero sum, and different weightings benefit one group over another. For example, "Asians" (the US does not differentiate South Asian [India, etc] from East Asians [China, Korea, Japan]) have very high test scores in the US. If you base your college admissions just on taking the people with the highest test scores, you would get a massive over-representation of Asians. Schools have decided that isn't what they want, so they take people with worse test scores in order to "balance" the student population.
This infuriates the people passed over in the name of "balance", naturally.