It saves you work and the student the ego-deflating experience of looking at a paper filled with criticisms. You should put the grade on the last page of the paper. A student's grade is not for public consumption. Points in style include basic grammar and spelling. Although most graduate students are good writers, it is also true that many cannot remember why they write as they do, thus it cannot hurt to review a writing manual to refresh oneself of the rules.
Style also includes sentence and paragraph structure, vocabulary, and organization.
Organization includes a strong thesis, the introduction, conclusion, and a logical organization of the points to be argued. Points in content include a familiarity with the material, the incorporation of examples to illustrate or support the argument, and identification and analysis of the issues. A good paper should be strong in all points, but exam essay questions are often marked with a degree of leniency in the style. No doubt, your course director will clarify any confusion about the standards expected of the students. The UVic Learning and Teaching Centre makes available videos that offer helpful teaching tips.