As I've discussed in my "why I care so much about your writing" page, most teens simply aren't up to college-level writing when they reach college, no matter how much they think they are; they're comparing themselves to the 3/4 of students who aren't writing well at all. I've had some students become rather miffed at me or down on themselves when I've let them know that all of their hard work in high school (usually with an English teacher they admire) still hasn't gotten them up to college-level writing. 

http://www.dailymail.co.uk/news/article-2203385/Less-American-teens-proficient-writing-skills--allowed-spell-check-computers.html 

http://m.heraldsun.com.au/business/worklife/are-poor-spelling-and-bad-grammar-affecting-your-career/story-fn7j1dox-1226466140522