Signed a payment contract for $X amount/month for dance classes and now they want that amount per week. What legal ground to I have to refute this?

I don't understand the basis for the dance school's claim that you should pay $x per week.  Do they claim that there is another agreement, written or oral, in which you agreed to make weekly payments?  Do they claim the agreement you have was a mistake?  Do they claim that you defrauded them in some way or that the agreement you have is not valid for some reason?  Absent some reason why the contract you have calling for payments of $105 per month is invalid, the dance school may be able (depending on the terms of the agreement) to cancel it, but they cannot compel you to pay more than that.