Last semester I wrote a brief (10 page) paper on the artwork of Degas inspired by the ballet of the Paris Opera.
Here's a bit on how\why many entered:
"Most of the girls came from parents of the poverty-sticken lower-middle class who enrolled their daughters at ages seven and eight with the hope that they would be hired by the company in a few years’ time and help bring money into the household. For three years these children, underfed and in cut-down clothing, would attend lessons from nine o’clock in the morning until four in the afternoons in the hopes that at age ten or eleven they would be good enough to begin earning an annual salary for their work. Sadly, most of the members were expelled with no other words while only a few from each class were hired by the Opera."
I also found that after the girls were hired it was actually their mothers who would find "gentlemen protectors" for their daughters. So not only were these girls exposed to forced labor in order to (hopefully) help support their families one day, but they were pushed into prostitution by their mothers.
"Unfortunately the families of the dancers were not the only ones to rely on the money of these wealthy patrons, but the Opera relied on them for financing productions as well, giving them a great deal of power. They bought their way into anywhere they chose to be in the Opera house (even changing rooms) at any time, could have scenes added to operas and ballets, and had a say in what would be chosen for the season."
How I viewed Degas's work was totally changed after I learned those dark, looming figures were not actors or managers... Poor little girls.