If you're a girl and if you're usually quiet in class:

See, when my teachers do that (and if I know the grade is a mistake), I just go into their rooms and try to rationally explain my concerns to them.

Then I let my face get a little flushed and get my eyes a little watery-- NOT any actual tears, fake or otherwise-- just enough so they can tell the waterworks are fully functioning and ready to leap into action.

Not only do they suddenly commend you on your commitment to your education and give you all the points you wanted, they feel bad for getting you the wrong info, and you're on their Good Side forever and ever and ever.

This works with both male and female teachers (especially female teachers and older male teachers that don't really give a crap anymore since they're retiring in a year). I encourage you to try it out! Unless you're a guy. That would be bad.