Listed below are a few of the online paper rating/"grading" tools that are available. Note that these tools don't "fix" your paper, but they do offer suggestions based on tallying your paper for various features that tend to be used in successful writing.
They usually offer grammar and style advice for the paper and can sometimes do a preliminary plagiarism check (usually checking everything that isn't in quotation marks to see if it is a direct copy of something else that exists online). They sometimes also offer additional, more thorough plagiarism checking with paid membership, but I can't attest to the quality or value of these added tools because I've never purchased them. Still, I find the free services to be very useful. You can chose your grade level and essay type for some of these tools (such as PaperRater.com), which helps determine the "grade" that will be assigned.
The disclaimer is that this is, of course, not an official grade and can't necessary be used to determine what grade the teacher will actually give. I like to use it as a rough estimate of what I may receive if the professor is a generous grader. I use all these qualifiers to be safe because the computer is going to grade very differently than a professor, and no matter what the paper checker gives me, my actual grade can only be determined by the professor. Still, my skeptical view of the estimate has worked pretty well for me. One paper that PaperRater gave me a 90% on got an 84% with a teacher who was a strict grader. Another paper, which got a 99% from a generous professor, got a 91% from PaperRater. Overall, PaperRater seems to find that I don't use transitions nearly as much as it would hope, but I hardly get comments about the amount of transitions and I do actually use them frequently. It may be that I just may not be utilizing the transition words in PaperRater's list of pre-selected "good transitions." I'm sure that when I do take the suggestions from PaperRater, it doesn't ever hurt my grade, only helps.
There are useful suggestions on spelling, grammar, word choice, and style on PaperRater. Similar tools may also give a readability score, such as the HemingwayApp (http://www.hemingwayapp.com/) and ReadabilityScore (readabilityscore.com/), although some readability checking sites may not offer any grammar or style suggestions. The readability tests are based on different formulas that researchers have developed to calculate combinations of vocabulary and other factors.
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I'm just your average fictional creature, living in a swampland by the sea.