It's is a very interesting question whether doing well in HS or college is "harder". I think the level of intellect required to succeed in college (and hence get into a good grad school) is higher than what is needed to succeed in high school (and hence get into a good college). But in terms of difficulty of getting in, it may be more equal, or even swayed in the other direction.
My non-professional analysis hinges on the opinion that success in high school relies on different types of achievements than success in college. In high school, the typical top student is the classic well-rounded boy or girl who not only has top grades and test scores but also plays 3 varsity sports, is treasurer of the student council and is president of 3 student organizations, and ran local community service programs. Getting high grades in HS (except perhaps at competitive specialized private schools) is just not too difficult for those willing to put in the effort. To stand out intellectually, one needs to go way out of his way to draw attention to himself. Like cato88 suggested, science fair competitions is one thing. Some students take classes at nearby colleges while in high school, do all kinds of independent work and whatever BS... So many parents are starting their kids off at a young age to become star students…
There's very little chance for a typical talented, ambitious student in HS to go to a top undergrad college without playing the game since there just won't be enough chances to prove himself. Everyone takes essentially the same set of courses with minimal flexibility (either the AP/honors track or the non-honors track, and everyone going to college takes honors anyway). High SATs are just not going to impress many people. The math section is easy for anyone with decent skills. The verbal... mostly based on memorized vocab, is not relevant to the skills they need (more of a dork-detector than anything). Besides, so many students score high anyway. Imagine how many more 1600 SATs the top schools see from prospective physics undergrads compared to domestic 990 physics GREs.
But in college, people care much more about success in one's own field than any tangential accomplishments. Opportunities to shine are more naturally part of the experience. Most of the courses one takes are within his field, and they're far more challenging than "10th grade English", so the results are more important to grad committees. One can take very rigorous schedules to distinguish himsef, and double major. There are plenty of research opportunities available at most schools, along with relevant jobs like tutoring and TA-ing. Also, the standardized tests in college (specialized tests like subject GRE, LSAT, MCAT etc...) are far more relevant to one's merit than the SAT, and they're more difficult, which means getting a top score is a substantial achievement that gives you a great chance as long as the rest of your record is good. OTOH, most 1600 SATs are rejected from the elite undergrad schools.
Other factors may include the fact that a bachelor's degree is practically a prerequisite for gettting by in today's world, so there are far more people trying to get into undergrad than in grad school. However, there are also more spots. Also note how undergrad education is very expensive, so schools have an incentive to bring in as many students as they can handle. But in certain grad fields like physics, bringing in a student is a substantial cost and funding is limited, so that will naturally make things harder for us.
The prof I worked with in undergrad had a daughter who went to Caltech for physics. She was valedictorian of her high school (kudos) but she also participated in 3 research projects while in high school (WTF??). I wonder if having a physicist as a father helped any in her quest? How many times did you see research advertised in your high school? I never even heard of research… to us, research was a type of book report where you had to use the library… lol!
Clearly getting into the top undergrad schools usually requires some serious help along the way. The student and or his parents, fan club teachers/guidance counselors, must go well out of their way to get special opportunities. Whereas in college, those types of things still happen, but it’s still feasible for one to be a quiet success without drawing that much attention to himself. I took as many math and physics courses as I could, did great on the GRE, had a few fun side jobs, did an REU and turned out OK. Some students in the bio and chem departments were flying all over the country giving talks and networking and getting guided to “special scholarships” from the honors advisor at my school who loved them, and they did as well as I did… but at least I can say that “I did well” without stretching the truth.