As a college-level instructor who has actually been wrong (believe it or not!)... the best way to handle this is to:
(1) Meet with the instructor privately. Don’t call him/her out in front of the class.
(2) Bring sources justifying your position.
(3) Be open to the possibility that you, as a student, are mistaken.
Last year, I was teaching a senior level neurobiology lab course that has a couple prerequisite courses. One of the principles I was testing the students on was the idea that an action potential is considered “all or none”—either the signal fires or it doesn’t, but there’s no such thing as a small or large AP. So I asked them if a very strong depolarizing pulse was given during a neuron’s relative refractory period, would the AP be smaller, larger, or the same size?
The answer I was looking for was, “if the pulse is strong enough to depolarize the neuron, the AP will always be the same size.” Yet about ¾ of the class answered that the AP would be smaller! So, I told them I would give extra credit to the first person who explained why this was wrong.
One of my students emailed me and said that he was still very certain it was the correct answer, and he quoted a section of a textbook he had used in a prerequisite course explaining why. It turns out that during the refractory period, conductance changes slightly in certain kinds of ion channels, which means that the AP really, truly is smaller during the refractory period.
I had been told for literally a decade that there was no such thing as a “smaller” AP, and none of the other TAs who taught this course had ever heard of it either. It turned out that in one of the prerequisite courses the students take, this was a special case they learned about, because the professor who teaches it studies this phenomenon in detail.
So, I apologized profusely, gave all the students points back, and gave extra credit to the student who corrected me.