The problem lies mostly with style, not content.
It's not that the students lack good ideas, but rather that they have not yet developed the confident writing voice with which to express them. Even an essay about school uniforms can be made entertaining by a strong, unique voice.
The process by which that skill is attained is iterative and beyond the scope of any singular semester, but I think it begins with a foundational understanding of grammar and mechanics. Dry as they may seem, these are the tools with which every great piece of writing is crafted. Grammar needs to be presented not as a stuffy, boring set of conventions, but in fact the opposite: a way to artistic freedom. In the same way that a knowledge of music theory opens doors for musicians, being well-versed in grammar enables the writer to fully express himself with authority and legitimacy.
When you have mastery over the rules, you can write anything you want—and be taken seriously.