Without changing the values you used in your radio button field, you'll need a calculation field in students that returns a 1 for Paid and null for not paid.
Something like: cPaidflag : If ( RadioButtonField = "Paid" ; 1 ; "" )
Then a calculation field in Course Schedule can be set up that uses the count function:
Count ( Students :: cPaidFlag )
Will return the total number of paid students enrolled in a given class.