One of us is in the process of finishing her grading for the semester, and here's the top 5 list of exam-writing mistakes (so far):
(5) Reiterating the facts without using them in the answer. Especially for a page-limited exam, the only facts a student should mention are the ones that the student will use in his or her analysis. Even for non-page-limited exams, wasting space = wasting time.
(4) Incoherent answers. If the professor can't follow the structure of the answer, then the professor can't assign points for the answer.
(3) Making unsupported assumptions. It's OK to make assumptions that help the student answer a question; it's not OK to make assumptions that bear no relationship to the facts in the question, just to be able to demonstrate knowledge of a topic that the question doesn't itself raise.
(2) Failure to proofread. If someone has a take-home exam, it's useful to proofread it before turning it in. The worst mistake this semester has been someone who turned in an exam where the answer to question 2 was also the answer to question 3, resulting in zero points for question 3.
And the number 1 mistake?
(1) Failure to follow instructions. If a take-home exam has a 72-hour deadline, then 72 hours and 4 minutes (without a prior excuse) is equivalent to the failure to turn the exam in at all. And if the professor wants the exam file named a certain way and the student doesn't use that naming convention, then the student has thrown away points. Points are hard-earned enough; don't throw them away out of carelessness.
Why is #1 the most important? At least for students who intend to be litigators, all courts have rules: page limits, font sizes, deadlines, etc. The failure to follow the rules might mean that a filing gets rejected. And a rejection right before a deadline might time-bar a client's relief.
Oh, and one last thing: students who have earned a Bachelor's degree in any subject need to know how to communicate, so law students need to know, at a bare minimum:
(1) "Its" and "it's" are different words, and they mean different things; and
(2) It's impossible to make a noun plural by adding an apostrophe and an "s."