I think you need to take a look at your basic design here.
Is this a table of students or a table of classes?
Generally, assigning students to classes will require from 2 to 3 tables and the student records would be in one table while the check box field would be in a related table or it might not be a good idea to use a check box field at all for this.
Students----<enrollment>----Courses ( one----<Many )
Students::StudentID = Enrollment::StudentID
Courses::CourseID = Enrollment::CourseID
The assumption here is that a given course can be offered more than once in a given semester. If this is never the case, you can do it with two tables instead of three.
To enroll a student in a specific class, you create a new record in enrollment and assign it the student's ID number and the course ID number. Thus, a student can take many courses and a given course can be taken by many different students.
its a table of students.