Certain words just don't inflect for gender. "La personne" and "la victime" are always feminine, regardless of whether they actually denote a male or female. Hillary Clinton was "LE secrétaire d'Êtat" of the US. So, too with "le professeur", masculine even if referring to a woman. Now I know I'll be drummed out of the Académie for saying this, but "la professeure" is floating around out there in francophonie, but be aware that it's not prescriptively standard. If you're in a French class, try it out on your teacher, and if his/her eyes expand to the size of saucers, then you'll know not to use it. :)

Juste pour compléter, je confirme que l'on peut utiliser le féminin "une professeur" (au moins à l'oral). On dit alors "la professeur de français".