Part of me thinks the #MeToo movement might have caused a surge of high school boys to start identifying as conservative. In some of their eyes, the movement might have been "anti-male". This might also explain why that number is decreasing as of late; with #MeToo dying, high school boys have fewer perceived gender-based reasons to oppose being "liberal".
Most of the 2010s was dominated by catastrophically terrible feminist messaging that was pushed heavily by the entertainment industry and most left-of-center political parties in the west. It was a big mistake.
I say specifically the #MeToo movement because the data indicate there was a sharp decline of support for liberalism amongst high school boys from 2016 to 2018. The #MeToo movement started in 2017.
I get what you're saying. The MeToo movement in a lot of ways was the culmination of that terrible feminist messaging that I talked about.
While there were examples of guys who were unfairly accused to dismiss #metoo as “culmination of that terrible feminist messaging” is kinda unfair as it did a lot more good than harm