Populate Junction Table Related To 3 Tables

I explain , I have 5 tables :

Class ( classID , classname,... )
Student (studid , firstname, lastname , classID ,....)
Course( courseid , coursename )
Term ( termid , termname , begindate , enddate )
Score ( scoreid , studid , courseid , termid , score)

Note :
1) A class can contain one or more students (one-to-many between Class and Student tables)

The table "Score" is a junction table between three tables : Student , Course and Term because it contains three foreign keys ( I could use a combination of 3 foreign keys to make a primary key ! ) .

The tables ; "Class", "Student", " Course ", "Term " already contain data for each table I created a data entry form .

My biggest problem is how to create a form to enter students' grades or scores for each student that belong to his class.

I do not know how to do it especially since the idea is that :

On a form I would like to use ComboBox to select a class that displays student's list from class selected and a ComboBox in same form to select course and another ComboBox to select a term and then enter grades or scores for each student

This is my general idea to enter students' grades. The rules are :

A class contains one or more students
Each student takes one or more subjects.
Each student gets scores for each subject and each term(quarter)