In order to avoid unnecessary problems, you must be able to answer these and similar questions on the first day of classes. This means that you need to decide in advance, and in many cases in conjunction with the instructor of record or your supervising instructor, what the answers are.
Most departments have policies on grading and issues related to grading.
Although some of these matters may seem trivial to you, if not resolved properly they can cause many problems and great headache later in the semester. Once you resolve them, you must decide which among them to include in the syllabus and which to communicate to students orally. One the one hand you want to set all important policies as early in a semester as possible. On the other hand, you do not want to make rules so inflexible that they cannot later be changed if necessary. The single most important think connected with grading is how the final grades will be assigned.