Tables :: Updating Table Data In Event Handler

I am working with MS Access. The database has 2 tables.

-Parent and Student and ParentID is the Primary key as a parent may have multiple Students
-There is a form that lets me add students for a particular parent

One of the fields in the Parent Table is FeeDue. I added a field in the Parent Table called NumOfStudents..What I want to do is as follows: When a student is added on the Student Form, I want fee to be calculated automatically for display AND update the FeeDue field in Parent table. Event handler executed when a student is added (checkbox clicked)

OnClick()
{
Read NumOfStudents from Parent Table

if(student_added = true)
NumOfStudents++
else /* This is to cover student withdrawl*/
NumOfStudents--

if(NUmOfStudents = 1)
Fee= 400
else
Fee = 500
}

student_added check box is on the student form NumOfStudents and FeeDue are fields in Parent