
It's not really that girls just don't want to be in there, more that they are so fixated on their own image and the weight room has slowly become stereotyped to become the "mans man" room. It's also not that girls don't go to the gym at all, more that they avoid using the school weight room.
The one time I saw the whole girl's soccer team in there, I started hearing complaints from all those hard core lifters about the large crowd and waits to use the equipment. The social disruption created when girls begin to enter the school weight room shows why this are has become a gendered section of the school. All the large football player sized people in weight room make it awkward for girls to go in there; these people create a sense of intimidation, which shies away anyone new to weight lifting. In a sense, the weight room is also sectioned off to only the already "big guys".
To resolve the social stigma around the "ownership" of the room seems to be a very difficult problem. The only thing between girls and the weight room at this point is their own conscious; they need to get out of the mindset that they will get judged for using the largely masculine room at our co-ed school.