It should be because of the huge number of standardized tests now given in high school and the push to keep kids from dropping out but there has also been the addition of both internet posting of missing assignments, internet help, after school tutoring and greater distribution of student performance to parents than used to be available so this is balancing the greater demands put on high school students. also AP classes are more difficult than college classes BUT you have much much more help from the instructor, so this is really good preparation for college.
So the major factor now in college is the same as it has always been - the ability of an 18 year old to self motivate, avoid partying during the week, get to bed early enough so they don't sleep through morning classes and so on. But this is somewhat balanced by the number of students who are attending college while living at home and the number who are doing classes online from home and the increased number of adults who have gone back to college and who will study 24/7 and "blow the curves" as we used to say. also of course it depends on just how strict college admission is these days. When I went, I had never made a grade below a C in high school and that was when my family moved in March and I went into classes that were on different material in a new school. My freshman year of college I failed a class, first time ever, because I had inadequate math background, but went back, took the math, repeated the class and got an A.
So I would say in general, high schools are doing a better job with college prep students than they used to and I don't think typical college freshman instructors have done a lot of improving, so for the college bound, boarding on large university campus away from home student, it's still probably more likely that they would fail in college just because of the lack of support, the huge classes and the impersonal atmosphere in college.
Ivy League schools are different because of their really high entrance requirements. If you get in you can probably pass unless you just don't go to class or take exams.