If you can prove you submitted them to the principal, they are obligated to correct it. Which/ since it sounds like she finished grading them but didn't input the grade itself into the system- shouldn't be a problem. See if it happened with any of your other class mates, and if so, go as a group to the principal. Worst case scenario is that the principal or dean is a scumbag and refuses to change the grades and you all can then go to the news- or whoever enforces school integrity in your state- with the story. Best case scenario, the principal does their job and makes sure the grades get corrected, and also forces the bird-brain to fly the coop for having her priorities so flocked up and being so bird-shit crazy in general.

Did she tell you that was her reason for neglecting your extra credit and her job or are you assuming? Either way, bring this incident to the principal or a counselor because you're most likely not the only person she has done this to. Hope everything turns out okay.