Admittedly I know little about how puberty works with girls. But isn't having explain what wetness between a woman's legs means, to highschool girl, roughly the same as having to explain what an erection means to highschool boy. Because I'm pretty sure, that higschool boys would be well acquainted with erections, even if they were somehow so naive they didn't know what they were for.
So, arousal isn't the same as masturbation, but I've only seen survey data on masturbation (yes, this exists; self-reported age at first masturbation) For boys, by the end of age 13 over half (60% even) have masturbated to orgasm, and by 16 90% have. For girls, age 16 is their 50% mark, and they never reach 90% -- at least 1/6 in the survey say they had never done so even into their 20s.
So it would seem that female sexuality is much more spread out across ages, relative to visible puberty, than male sexuality is. And big gaps in not just experience but 'awakening' are actually pretty plausible.
Not statistical, but reports on reddit are similar. Some before 10, many not until well past 15.
Suuna being clueless until Ruka hits her with a stick seems fairly plausible.