Not at all, it is anything but a laughable matter, especially for your son who has been the victim of classroom bullying.
Unfortunately, there is nothing to counter. The bullying is a matter for the school to deal with, unless any physical contact has been made on your son. Then in that case it is an assault.
I would suggest agreeing to pay for the damaged item, after all, it was damaged by your sons actions, albeit in retaliation for being bullied. Although he did not intend to cause the damage, it was reckless, and that could mean if they chose to, report the damage to the police. Your son could also be suspended etc.
So agree to replace the calculator.
Then raise the mater with the head, demand a formal meeting to discuss the matter and why it happened and demand that the school take action to ensure the bullying stops. If any of your sons property has become damaged when they have tossed his case on the floor then make the head aware of that and ask that your sons damaged items are also replaced by those responsible.