There's no reliable statistic on the number of kids who quit school *because* of homework. While homework can be a contributing factor to students feeling overwhelmed and disengaged, it's rarely the sole reason a child drops out. Dropout rates are complex and influenced by many factors, including poverty, family issues, learning disabilities, bullying, and lack of engagement with school.
Studies might show a correlation between excessive homework and negative attitudes toward school, but attributing dropouts directly and solely to homework would be inaccurate and an oversimplification.