filling a M:N table foreign keys

[siaswar]

I have simple db with 4 tables: class, book, student and studentbook. studentbook has st_id and book_id as foreign keys and an extra field for result. book has some book for classes. I just insert new students. I'm quite beginner in db and don't know how to fill studentbook. should I fill it (just foreign keys) when filling student table? or it can be automatically? I don't know the routine. I should insert students and result in different times.
Extra information:
I use visual studio 2010 for programming and access 2003 for db. for user-friendly reason I want a datagridview that shows students and result in on table to insert result for each student. I got some success but it's not quite update but inserting action, that my insert two or more rows for a student in studentbook table cause clicking of updating button.
Please any idea or simple code? thnx