Then you have to relearn the five paragraph essay for the GRE, and it's worse than the first time.
One of the best ways to really 'get it' is to make outlines of things that are already written, to learn how it flows from one topic to another and back, and if it actually works. You can also reverse outline things you've already written and get a good idea of what's happening or missing and how it could change.
This is a decent resource, if a bit unorganized: http://harvardwritingcenter.wordpress.com/
"Lens essay" is worth a web search, if you're curious.
You don't need that structure, but for what they are looking for, it does often make your life easier.
Then again the essay part is the most pointless part for those pointless tests.