Opinions and theories!

My experience indicates that it is entirely possible for students to graduate from many (most?) high schools in the United States without the ability to write well.  Furthermore, it is possible for a student to graduate high school at the top of his/her class, with a 4.0 GPA, even, and write a truly un-readable college application essay.
I'm never sure how much "insider information" I can/should share about the college admissions process now that I am in the know.  It's fairly fascinating (to me . . . you might be bored out of your mind already), but it's also a subject that requires sensitivity.  The college search process, the way schools select students, and the ways students and parents react to rejection from colleges are all deeply personal.  Over the past year I've fallen into a pattern of being candid with family and close friends, but fairly discreet on the internet.  (This, I think, is a good philosophy when it comes to sharing just about anything.)
But I'm guessing you already know that there are all sorts of problems facing high school graduates today, so hopefully I am not crossing a line in corroborating that story with personal experience.
I will say that I have no concrete idea why students struggle to write well, but that I do not blame their teachers. The teachers I know work hard and care about their students deeply, but have huge numbers of students to work with and little to no time for grading and providing feedback.  Teachers are amazing, intelligent people who would give anything to help their students realize that they are also amazing and intelligent. 
I do have a few vague theories why students struggle to write well, though.  They are (in no particular order):

1. Students in high school, middle school, and even elementary school (and, if The Nanny Diaries is to be believed, Park Avenue pre-schools) are wildly over-committed.  They are expected to be part of clubs and teams and scouts and classes and lessons and youth groups and volunteer organizations, and to do all of this while keeping up their grades.  I posit that you can have a worthwhile youth without being involved in ten different organizations, and I can say from experience that you do not need all of these to be admitted to college.  I tell students that I am looking for a few meaningful extra-curricular activities that they wish to continue in college.  Some students list a dozen clubs and teams in the activities section of their college applications without checking "I plan to continue in college" for a single one.  This leads me to believe that they are not actually happy participating in a dozen extracurricular activities, but that they feel they must.  I'll admit that I'm impressed when a student is class president, captain of the soccer team, an Eagle Scout, on the youth worship team at their church, takes violin lessons, and works at McDonalds (and I see this level of extra-curricular acheivement quite frequently).  But I also wonder when they sleep, and I don't wonder when I see that their writing has suffered.  I feel like giving them a hug, a free pass to quit a few things (if they want), and a talk about how they are worthwhile without all that.  Some students are genuinely interested in all of their extra-curricular activities: Travis played 6 sports growing up, and loved every minute, and I always begged my parents to let me take more dance classes.  But regardless, over-committed students are sleep deprived and have little time for school.  And honestly, even though I was in Student Government and dancing and involved in my church, many of my friends were doing much more.  I am really glad that I had parents that told me that school was my job.  They are the only reason I spent my extra time on homework and sleep, and not on things like National Honor Society (you do NOT need this to get into a good college). 
2. The internet/texting/instant messaging, etc. have made it okay to write without regard for grammar, spelling, capitalization, and punctuation.  As my middle school band teacher always said (and yes, I quit band when I had too much going on), "Practice makes permanent, not perfect."  If you spend 3 hours a day writing sentences with no punctuation, your punctuation is guaranteed to be a complete mess when it actually counts.  It's like practicing a piece of music in the wrong key for a month, then going into a concert and expecting to play it perfectly in the correct key.
3. Students are forced to read terrible books in school, hence they don't think they love reading, hence they don't read for pleasure, hence they don't have a lot of examples to draw on when they are writing.  A friend of mine from college is having her high school students read the Hunger Games trilogy for English class.  I will never read those books because I have  no tolerance for violence anymore, but I am absolutely in support of her plan.  Get those students reading books they actually like!  The books I was asked to read in high school, with a few exceptions (Les Miserable!  Huck Finn!), were depressing and a little (to a lot) dry.  Give 100 high school students The Winter of Our Discontent as an example of the most worthwile books of all time and I guarantee that most of them will come to the conclusion that there must not be anything good out there.  I do believe that my love of reading has made me a fairly intuitive writer.  I know I am not some sort of writing savant (the truly terrible book I wrote in the third grade about Santa Claus is evidence of this), but I don't have to think much about subject-verb agreement and sentence fragments today because I have read hundreds and hundreds of well-written books.  And I know there are lots of good books out there for teens because those are the books I still gravitate toward today (they tend to contain less sexual content and violence, and are fairly easy to read). 
4. Students think proofreading/editing is boring but necessary.  Proofreading is actually kind of fun!  Years and years of dance have made me value a good critique when it is given respectfully (and when I have actually asked for it).  Most students know the value of critique and polishing when it comes to everything but their schoolwork.  I'm not sure how this mindset can be extended to writing in schools, but maybe it should involve candy?  What I do know is that proofreading is vastly under-utilized on college applications and application essays.  I'm fairly convinced that many students finish up their essays and never want to think about them again, so they don't.   
5. Maybe it has always been the case that the majority of people are not strong writers (How long have we been a mostly-literate society, anyway?  110 years?  Not that long.), but it is only now that we expect them to be.  We expect everyone to be brilliant at everything, and the reality is that is just not how we are made.  If our ability to succeed in college, the work force, and life was determined by our ability to throw and catch a football, I might be the subject of a blog about how America's youth are failing to measure up.  Maybe it is not students or education that are broken, but our expectations?

Those are my theories.  They are probably all wrong.  I will say this, though: I think that standardized tests (the SAT and ACT) get a bad rep as biased and completely useless.  While I am proud to work for one of the few colleges that allows students to choose whether or not they would like to submit test scores as part of their application, I still believe that SAT and ACT scores correlate fairly well to writing ability.  This may not be the case for subtle differences (a student with average SAT scores may be a stronger writer than a student with slightly above-average scores who has been able to afford a prep class).  But sometimes a student will have a near-perfect GPA and their writing sample will be full of grammatical errors, poorly constructed, and carelessly proofread.  This student might be able to eke out a strong GPA because of grade inflation, the relative strength of other students at their high school, help from parents, etc.  But they will never have a near-perfect SAT score.  Conversely, a hardworking student with a B-average from a competetive high school who writes well will often have strong test scores.  Without test scores and/or writing samples, the first student would have an advantage in the college application process, when the second student may be working part-time and taking tough classes. There are many exceptions to this rule, but I would like to put myself out there as someone who thinks that tests have merit.
Despite my skepticism, I still want to admit every student I come across, even the ones that struggle with writing (English many not be their first language!) and the ones with low test scores (they could have ADHD and it's just not reasonable to expect them to focus that long!).  At the end of the day, the actual person behind each application is much more important than whatever opinions I have about writing in American high schools and standardized tests.  I am grateful to have a job that reminds me of this every day.