Excellent advice above angel and megan, my 2c:
If you passively read essays you'll absorb some stuff slowly, but you might just as well (no, better) be reading something you enjoy.
I'd tend to use essays not as reading matter, but as tools to solve particular questions. For instance, if I wanted to learn how to structure intros, I might get 10 different high-scoring intros and break down how they structured them (do they respond to the prompt in the first sentence or spend an age getting to the point, how to they signpost arguments, how do they phrase their contention, etc.). Or you might look at structuring topic sentences, or how they avoid storytelling, or how they break down the prompt into paragraphs, or how they link their ideas, or how they vary their sentence structure, or whatever. Don't just read. Read with a particular purpose in mind, take notes, and then try to apply the specific thing(s) you learned to your next essay.
Re reading marked essays, I'd always do it with a document open or pen and paper. Note down ANYTHING that you can see that is relevant to you and your problems, like embedding quotes correctly, avoiding story-telling, how to structure intros, etc. There's often a wealth of valuable advice, though you'll have to disregard a lot of stuff that's specific to the writer's individual issues.
I repeat: take notes; don't just hope to absorb.