Writing in a convoluted, murky style is actually more difficult than writing more clearly and directly. A certain prose style feels laborious and inelegant. One imagines great effort went into it, but the wrong kind of effort, effort in the opposite direction. You can feel, in the prose, the tenseness in the writer's muscles. Good writing is not easy, but it should feel effortless to the reader. The reader might say, "Hey, I could do that" (even if this is not true.).
Having a high-school writer at home I realize that part of the effort is in building a sophisticated style in the first place. A lot of wordiness and pretention in bad college writing comes because the writer is still trying to construct a properly "written" sounding style. A more mature writer will be working more to simplify rather than to complexify, to write more as an eloquent person might speak.
I guess that's the difficulty of writing instruction at the late high-school / early college level. Is it time for learning or unlearning a vocabulary? Early academic writers know that academic prose sounds more complex and convoluted than their own, so they naturally move in that direction. The passive voice, for example, is a widespread in academic prose; no wonder college age writers overuse it sometimes: it makes them feel more sophisticated.
I think the best way of approaching this is through offering good models of writing from good writers. Look how simple some of the sentences can be, how the writer balances complex sentences with simpler ones.