If they provided some lessons, they are entitled to payment for those and would only have to refund the cost of the not-provided lessons:1) When there is a breach of contract, the measure of the compensation to which you are entitled is the loss you suffered; here, that loss is not getting X lessons (since you did however get Y lessons)--therefore, you are entitled to the value of X lessons only.2) Under the doctrine of "unjust enrichment," you can't get a service or benefit which is ordinarily for pay or charged for without paying for; having received the benefit of some number of lessons which were provided, you can't get that benefit for free and so they can keep the cost of those lessons.