@Cornonthekopp I think the issue is that there is no properly widespread sign-language. Or rather, there are
too many standards (comic mostly unrelated and just included as a pun). Not only does every language essentially have their own version (make note that it rarely is that the sign language
is in those language like how written text is in same language as spoken, but rather that languages, both spoken and sign, are semi-bound to regions), but even regionally in the same country you might find multiple different ones. Combined with how small minority uses them, not only will you end up unlikely to have use for any sign-language you might have learned as part of obligatory education, but also the spread of any single sign language is likely hampered .
Luckily there seems to be initiatives to standardize the sign languages taught, such that more ppl end up using the same (which makes hte above paragraph kinda outdated). Chief of which is probably how some nations pick one as their national sign-language, such that the required sign-language translation for national broadcasts stays consistent, which should encourage their spread.
But yes, just like it makes sense if the world agreed on a single language to use internationally and taught it to everyone in schools (luckily
french german english seems to have managed this somewhat by itself
back then up until ww2 nowadays), it would also make a lot of sense if they agreed on a single sign-language to teach everyone. Until then however, I don't see any reason to teach sign-language to everyone (lack of use will just make ppl forget it all anyway, like my third and fourth languages), though I do agree it should be encouraged as electives for third+ languages (second obviously remaining english as always). Like you said,
there should be more options for it as a class.