1, Research after research shows that girls and boys are equally capable at math (around the world).
2. If teachers think that boys and men are better at math (whether male of female teachers) girls will tend to do poorer in these math classes (girls tend to give up).
3. In societies where girls in general are seen to be less able in math, girls will do poorer in math overall.
Here is my thinking. If teachers notice that girls are not doing well in math then they MUST DO SOMETHING. Maybe talk to the girls, maybe change teaching styles. maybe change classroom dynamics, maybe change assessments (oral might be better?) WHATEVER. Please stop failing girls when they are smart enough to pass. AND Stop the cycle.
Here is a great article: https://thesocietypages.org/socimages/2013/03/07/the-truth-about-gender-and-math/