American society is committed to the idea that almost everyone should graduate from high school, including people with IQs in the 80s and 90s, who are just as numerous as the people with IQs in the 100s and 110s. You can give someone with a 90 IQ a high school diploma, but you cannot make him "fully prepared for college", unless you remove serious intellectual demands from college work.
Keep in mind the term 'college' in the U.S. means vastly different things. Both research Universities and community college and many schools in between are considered college. There are vastly different expectations for getting into these schools. Part of the problem is we have turned what used to be vocational school into 2 year AA degrees at colleges.
The level to graduate from H.S. is really very low. In CA the "exit" exam is given to sophomores and only requires a freshman level or math & reading. And the students are required to pass 4 years of English, but it doesn't need to be college "prep". Most universities I know require a higher level of math & writing/reading. A H.S. diploma does not mean one is ready for university.
