Re. the signs themselves, I’ve seen those for a long time around here, and I’m not sure they say anything about the moral character of teenagers (or even students). They do say something about the moral character of adults. If the sign said Only 3 black people at a time in the store, you would probably respond very differently. Or even Only 3 black students, even though that sign would be strictly more permissive than Only 3 students.
Then again, maybe this really is about schooling (which people can control), rather than age (which they can’t). I wonder if a group of kids in the store could offer the defense that they’re all dropouts.