There's not really any plausible explanation as to why so many left-handed students tend to skew towards boys, rather than girls, other than social contagion.
When my parents were kids, there were no left-handed kids. Social contagion is the only explanation for as to why there are suddenly so many left-handed kids today, especially since many of them are boys and not girls.
But the adult demographic of left-handers doesn't have, and didn't have, a sex ratio skewed in the opposite direction to the youth demographic. So how is this a relevant comparison?
When my parents were kids, there were no left-handed kids. Social contagion is the only explanation for as to why there are suddenly so many left-handed kids today, especially since many of them are boys and not girls.