I have enrolled for ...

I have enrolled for swimming lessons for my child for this term,she only went for 2 lessons in November,but they are billing me fees for the whole term,I asked them why did they allow me to enroll in the middle of the term,why must I pay so much money if the child only went for lessons twice and they are closing for December?they said long as the child is registered I must pay, they keep on sending me statements and messages that I should pay outstanding fees,