The problem lies with the measure of damages. While more complex, the basic difference between tort and contract damages is in tort it is to be made whole and in contract it is the benefit of the bargain. You seem to be trying to measure by the tort standard and not the contract one.
These "expectation" damages can sometimes be very hard to measure. To deal with this, many contracts list the amount the parties would feel they expect in case of breach. This is called liquidated damages. Sometimes liquidated damages are way out of proportion to the reality of the contract. If so, the courts often find they are a penalty and not a reflection of contractual damages and reduce them.
In your case, if the penalty was twice the amount they paid for classes, then I suspect the courts would reduce or deny it as a valid liquidated damages clause. Here it seems like they are asking for half. Since I've seen full repayment for tuition for leaving too soon after completion contracts enforced by the courts, I suspect this 50% measure would be enforced.
Finally, I've not seen the pro rata argument work. It doesn't seem like it should, but I can't think of the principle of why it shouldn't off the top of my head. I just know I havene't seen a case of this type use it successfully. However, I don't read a lot of cases here regularly and could be wrong.
When you are a Bear of Very Little Brain, and you Think of Things, you find sometimes that a Thing which seemed very Thingish inside you is quite different when it gets out into the open and has other people looking at it.
--W. T. Pooh (aka A. A. Milne)