Dropout rate is not relevant, because we're only looking at high achievers and we believe their dropout rate is negligible. There are roughly equal numbers of boys and girls graduating high school (technically, there are more girls!), but there are many more high-achieving boys than girls. Boys are much more likely to excel.

This is not a statement about the group average, but about the group variance. The SAT is not a good tool to use to assess group averages; I already mentioned this.