It is true that it will make one's passage through society easier if they are able to reproduce standardized spellings. Insofar as this sign is a signal that teachers are failing to equip students for life in that way, sure, it's concerning.
Spelling is different from other domains of education like mathematics, however: an explanation of "correct spelling" is not to be found in natural law, but rather in a Katamari ball of historical contingencies of chance, intervention, and everything in between. (Why'd we keep the _-ough_ in words like _through_? What if the Simplified Spelling Board had succeeded in its aims and totally reformed English spellings? What if Scots, not the English, had had linguistic dominion over the United States? Etc.) Moreover, in large part because "correct spelling" is derived from one very peculiar and privileged variety of English that not everyone speaks, your ease of access to correct spelling is going to be facilitated or hindered by facts about your SES, race, and ethnicity.
This is why I don't really care when I see a spelling error anywhere, unless genuine confusion results, or stakes are high enough that adherence to a rigid standard must be maintained. In many ways, nothing is gained from imparting it to a small child compared to something like basic arithmetic. And moreover, in some contexts I think it's not unfair to call an evaluative insistence on standardized spelling a vehicle for linguistic and therefore ethnic chauvinism.
I understand your original reaction--having proper spelling is a display of fitness to teach, in the same way that, say, having won some Kaggle tournaments is a display of fitness to do data science. But spelling is a rather peculiar thing to insist on, and I'd have been much more interested if the teachers had been writing out multiplication tables on sandwich boards. Spelling is sociologically thorny, and there are bigger fish to fry.