I think you've kinda landed right on it there, rog: They don't teach you this in school is a hoodie for freshers, and really only for freshers. Even by the time people get to second year in college, what they did or did not teach you when you were in school is largely irrelevant, and by the time you get to my age you can't even remember what you did or did not learn in school. So it just strikes me as a hoodie for 18 year olds and nobody else.
It'd be like if I had a hoodie saying "they don't teach you this on Matlock"... ok fair enough, maybe they don't, but what's it to me? I'm not 70, I don't watch Matlock, so why put that on a hoodie?
likewise, I'm not in school, I'm not even just recently out of school, so what's it to me whether they teach either kayaking or UCDCC-style behaviour in school? (and that's not just because I'm 25, I reckon I could say the same thing if I were 20).
on second thought, can the slogan be "they don't teach you this on Matlock"? ooh, or "they don't teach you this on Bear Grylls: Born Survivor".