You also need to adjust the Wiki figures because the page also says that more than twice as many offenses are reported against girls than against boys. So 96% of those who abuse girls are men and almost 20% of girls report being abused. Between 14-40% of those that abuse boys are women and less than 10% of boys report being abused.
Say 100000 boys and 100000 girls
19700 girls abused, 18912 by men, 788 by women
7900 boys abused, 5767 by men, 2133 by women (midpoint of 27%)
Men: 24679, Women: 2921
89% of abuse by men and 11% by women
So at a population level both boys and girls are more at risk from men, but girls are far more at risk from men than boys are. So if women are in short supply, girls should be supervised by women and boys by men.
I would suspect however that the decision isn't so much about the real risk and more about the perception of risk (ie 'what would the parents think'), plus a feeling that the risk might be offset by the view that male role models are a good thing for boys and to be encouraged.
Alternatively the school just have a traditional mindset and haven't really thought much at all.
Also it's quite difficult to track the source data that the Wiki piece is using, lots of references of references, some repetition and I think it's quite likely that the numbers might come from different sources too.