For 99% of the high school students I've worked with, their essays would be better if they had to pass Microsoft's spelling and grammar checks before they were submitted. The exceptions were all exceptional enough that they knew that they no longer needed to listen to those automated filters.
Company B will correct their software, but meanwhile Company C has come along and introduced a sophisticated analyzer which is able to grade the presence and quality of the logical argument being made in the essay. Company C will demonstrate how Company A and B's software grades as "Excellent" an essay which is correct English and isn't gibberish but makes no logical sense.
After an indeterminate number of iterations with the software getting better and better at finding issues, the only way to game it will be to write a great essay.
At least for this training set, my algorithm rewarded the length of the essay most of all (something like 65% of the total prediction). The only other significant factors were misspellings and prevalence of certain parts of speech.
That model matched the accuracy of human graders and several commercial essay grading packages.
Students reverse-engineering comparable algorithms won't necessarily have to write well to score well.
Or, if it's slightly cleverer than that, certainly you could defeat it by producing a single, perfect-length, stylistically fantastic essay... which would be regurgitated word-for-word regardless of the subject matter.
I think, mind you, that software does have a place in analyzing student essays. If I could scan in an essay and have it spit out a word count, highlight any spelling errors or potentially problematic turns of phrase, and [most importantly] analyze for plagiarism, that would be valuable.
in contrast, i suspect algorithmic approaches can be gamed more easily because they don't adapt in the same way. they're not solving the hard ai problem; they're grading essays (currently) written for a human reviewer.
for example, what happens if a child learns an existing text by heart and then substitutes appropriate nouns and verbs to suit the context? say they learn "We hold these truths to be self-evident, that all men are created equal" and then, for an essay on their favourite pet, they hand in "We hold these kittens to be furry, that all kittens are created hungry". That's good grammar; it's got suitable references to the subject; it's clearly nonsense.