A family of Microsoft relational database management systems designed for ease of use.
Ok, there may be a issue of terms here. When I talk of the grade, I am referring to the score earned by the student for that course for that period. In your table it is called Grade. But it can be called score, marks, or whatever. You referred earlier to "in Mathematics scores 72/100", so this would be the 72.. If the field in the table named Grade is not for this value, then add a field for it and add a control bound to it.
And by the way, we use grade in both ways. It could refer to a score on a test or for a course or it could refer to a level of education.
Now, you can pre-populate all the records if you want. What you want to do is create a query that returns all the student in a Center. Add the Course table without a join. This should give a listing of all the students with one record per course (so each student would have 4 records.) You would then add a column for the month and turn it into an Append query. When you run that query it should add a record so you can enter the marks.