You'll need at last two tables, possibly three to do what
you want.
One table would store general information about each
student or as a minimum store each student's name. I would
suggest including a field called something like StudentID.
Another table would store the names of all of the classes
that could be selected. I would suggest having a field
called comething like CourseID.
Another table would link the two of these tables together
and would store the StudentID and all of the CourseID's
that a particular student is enrolled in.
What you end up with is something like this:
Student Table
-------------
StudentID FirstName LastName
01 John Doe
02 Tom Miller
03 Sandy Duncan
etc....
Course Table
------------
CourseID CourseName
01 Geology
02 History
03 Biology
04 Meteorology
etc....
Enrollment Table
----------------
StudentID CourseID
01 02 (Shows that John Doe has History)
01 04 (Shows that John Doe has Meteorology)
03 01 (Shoes that Sandy Duncan has Geology)
03 02 (...etc...)
02 01
02 02
I would suggest making the primary key for the enrollment
table a combination of StudentID and CourseID.
You could make a main form for the student information and
then have a subform to choose which courses the student is
enrolled in. As each student record is chosen, the
appropriate courses that they're enrolled in will fill in
on the subform. As far as selecting the courses, make a
combo box on the enrollment subform that queries the
Courses table.