When I taught college, one of the most frustrating things for me was how ill prepared students were to write college level papers. Many high school students are not graduating with skills that professors expect them to understand.
This results in both frustrated students and frustrated professors, and with grades that aren't as high as they should be. Unfortunately, this also sometimes results in lowered standards, as many professors give up after reading enough inadequate papers and don't require elements of paper writing that students really do need to know. Of course, this hurts no one but the students.
In my experience, one thing many students are not learning adequately in high school is this: how to cite sources correctly. Students need to come to college with an understanding of how to cite sources within the text of a paper and in the bibliography. They need to be able to use APA, MLA, or Chicago style-- and they need the understanding of style guides in general to be able to figure out how to use unfamiliar style guides.
I can't tell you how many bibliographies I've seen where students had no clue how to cite information correctly. The most frustrating thing I came across frequently was the citing of web sources with just a URL.
Another thing that students often have no idea how to write is a literature review. This is a more advanced skill, but students really ought to have some understanding of how to do this before college. Here's some information on how to write a literature review.
A final skill that students often lack is one of the most basic ones-- how to write an introduction. Students: first impressions count, so you need to master this skill!
Here's some more information about how to write a college term paper.