Just a thought,,,
Had you been properly prepared to take these exams, which would include having brought your own $10.00 calculator, you never would have had to borrow this girls $150.00 calculator and you wouldn't be in this position right now.
So you can look at it a couple of ways.
One, as mentioned above, you placed yourself in this position by way of your own irresponsibility. Learn from it, as this is one of life's lessons.
Two, did your having a calculator to use for this test, provide you with higher test scores?
If so, great, because then you did gain some value out of purchasing a new calculator for the person who was able to help you score higher on your tests. Now she wants to be compensated for helping you.
There is no free lunch in life,,, another one of life's lessons.
As far as you wanting a legal defense, you can simply keep your mouth shut, as the burden of proving you are responsible for breaking her calculator, will be on the other girl.
She must prove, by a preponderance of the evidence, that you are responsible. Just saying you did it would not have her prevail in a court of law.
If you were found to have done some damage to her calculator, then you would have to make her whole again. In other words, if it was a used beat up calculator, you wouldn't have to pay her what it cost when is was new, but what it was worth when you damaged it.
In your defense:
Calculators are usually pretty tough little gadgets. As you stated, the solar panel had some obvious signs of damage, yet it still worked. What could have caused the damage? How could you have done it during a test in a controlled area?
Does it sound logical that you would have caused the extent of damages that were done to this calculator, in the couple of hours you were in possession of it while taking a test. Or is it more likely that this girl caused the damages while she had possession of it for weeks, maybe months and transported it to and from school and wherever else during that time?
Would you have had to slam it in a car door to accomplish the degree of damage you're being accused of doing, or would just sitting on it be enough abuse to cause the damage?
How old was the calculator and is it under warranty?
I dropped a five dollar calculator off a roof one day. It hit a concrete driveway and exploded into several pieces. I put all the pieces back together and it still worked!
Wouldn't you expect a calculator that cost thirty times as much money, to be more durable?
Why did this girl wait three days to accuse you of damaging her calculator? Wasn't the damage obvious enough for her to have noticed it when you returned her calculator?
BTW, I'm no law expert,,,,, I've just abused a lot of calculators in my time,,,,