Should a girl remain in a mixed-gender work environment where she is negatively affected, or should she leave it fearing falling into the forbidden due to free time?

The fundamental principle for a woman's work is that it should be within her home, raising her children. It is permissible for her to work outside the home under certain conditions, including avoiding intermingling and causes of temptation, and anything that compromises her religious duties. Therefore, the work you describe, which contains many enticements and intermingling, is not permissible for you to continue. Instead of falling into the forbidden due to idleness, you can avoid it by creating a productive atmosphere at home, or through homeschooling, and attending beneficial seminars and study circles.