Combobox - list only remaining values available to group

[Richard]

Suppose ClassStudents has Class, Student, ... and a student is to be
replaced by a different one in a different Class.

If the student is shown in a combobox, how would the row source be
specified to only list students that are not already in the class?

select Student
from AllStudents
where Student not in
( select Student
from ClassStudents
where Class=<... Class of current row ...>
)

The ...class of current row... being the tricky part.