The first thing I look for in an essay is a unified argument. Is there a central point to the essay? And is it a point worth making? Aristotle claims, in his Rhetoric, that we do not dispute about sunrises and solstices. In other words, if the thesis is something everyone would agree with, it's not worth reading or writing. The best thesis is one where strong arguments, convincing to an audience, can be made on both sides of the question. The thesis also needs to be sufficiently narrow to be treated in the space allowed . Don't try to solve world hunger in 500 words (if it were that easy, people would no longer be starving) or write about "Shakespeare" or "the Victorian novel". Instead, choose a topic like "the language of flowers in Othello."
Next I look for how well the thesis is supported by evidence. Are there multiple types of resources for development and support (categorization, comparison, examples, deduction)? Does the essay actually prove the thesis, and respond to possible objections to the thesis?
Next, is the essay well organized? I want to see that there is a reason for paragraphs being written in a specific order, with clear transitions and explicit signalling of the relationship of every paragraph to the main thesis.
Finally, even at the 11th grade level, I want a certain degree of mechanical correctness. I's also like to see some variation in sentence structure (compound and complex sentences as well as simple ones).