More and more females are filing false charges of sexual harassment. Very few false charges are made. In fact, the majority of females do not file a charge or complain if they are harassed. Discussing sexual harassment is embarrassing, and people who report harassment say that doing so evokes many of the same bad feelings as being harassed. It is worth noting that filing a false charge of sexual harassment would in itself be sexual harassment.
Most sexual harassment occurs in classrooms - especially females being harassed.
The professor may realize that something is happening because of behavior changes, lowered grades, etc., but may not know that the changes are due to sexual harassment. Sexual harassment can be very subtle an hard to detect. Or the professor may believe that harassment is occurring, but may not know how to respond or may just accept it as a part of college life. Most sexual harassment cases are not reported. And, like rape survivors, they are often blamed for bringing it on themselves.