A family of Microsoft relational database management systems designed for ease of use.
OK, first, I don't really see the need for the first subform, but it does no harm to be able to view a list of students for the selected Center.
However, editing the Recordsources is not the way to go. As long as you set the The Linked fields properly for each Subform, then they should reflect the Selected record on the main form. did you use the subform wizard to embed the subforms? Did you select the Centercode as the linking fields?
As for entering the grades, You do thay in the second subform. You should have a combobox to select the Student, you already have a combo to select the course and month. Now you just need a textbox to enter the grade.
I'm still not clear, is it 100 marks for the period spread out over each course?