First, to sort out the combo box vs list box question. A Combo box operates on only one field at a time, so it can’t update an index using a name, at least not without some Basic programming. But a LO list box can operate on two fields, like for example, a name field, and an ID field. So as Ratslinger has suggested, you need to be using a list box, not a combo box.
Next, I suggest you rename your ID in the student form, to Student_ID, to make it clear what it is. Then in the Attendance form, you probably want two ID’s. An ID for the record itself, that is auto-generted, and the ID that points to the Student table (a foreign key). I suggest you call the first ID, the auto-generated ID in the Attendance table, “Attendence_ID”, and give the 2nd id in this table, the exact same name as for the primary key in the student table, i.e. “Student_ID”, as follows:
Students table fields:
Student_ID (auto-valued primary key),
Attendances table fields:
Attendence_ID (auto-valued primary key),
Student_ID (foreign id), and
(I have found that good names for indexes, just makes it so much easier to see what is going on. I always name the table in the plural, i.e. Students, and the index in the singular, Student_ID, as it refers to just one student.)
Then update your relationships w/ Tools/Relationships, making sure that Students
Student_ID is related to Attendance
In your attendance form which edits your attendance table you want two fields: a list box to select the student, and a field to set the attendance date. The data of the list box should be for the Student_ID. The list box should use an SQL query which returns two fields, in this order, the student’s name, and the Student_ID.
If you need any more specific details, just ask here with a follow-up comment.