Why should we not hire you answer?
But use the concept- turn it around and answer with a strength. You might answer “Why shouldn't we hire you?” by emphasizing your devotion to task completion, and de-emphasize another quality that is not important to the job.
Why should we accept you as a student?
You have to be able to add to the community and be a part of that community of students. They want to see how you're going to fit in that community. This is what your essay should focus on. Lean on your experiences, traits, and skills, and talk about how you're going to be a part of that community.