Mr. martin claims that boys tend to be better at math than girls. what a ridiculous overgeneralization! my friend alice is the best math student in her class. so, it's clear that boys are not always better at math than girls.

I believe the answer is: Straw Man

Straw Man argument refers to a form of counter-argument that is not actually addressed the previous argument.

In the example above, giving one example would not actually address the initial argument (boys better at math) because the initial argument referred to 'average boys in general', not just one.