I once watched a documentary on unisex bathrooms, I believe it was called Bathroom sluts 9. It was pretty convincing.
But seriously, I think that there are certain "things" that both sexes do in bathrooms that they would not want to do in fear that a person of the opposite gender may be present and perhaps even witness it.
Also, with it being a cash strapped high school, you may find converting two bathrooms into one unisex one may be too much of a cost or them to bother doing it.